Lecture 15. Global computer network Internet

Lecture 15. Global computer network Internet

The need for information exchange and modern technological advances have made global computer networks an integral part of the implementation of cooperation programs between countries. Many computer networks have been created for scientific and educational purposes, for business, financial and economic activities, the implementation of joint scientific and technical projects and many other applications.

A network that can unite many networks and allow one to join the global community is the Internet. The Internet is a worldwide computer network that unites individual local, regional and global computer networks into a single information space. The word “Internet” is a tracing of the English name of this network – “Internet”, which is translated as “between networks” (“internetworking”). The Internet provides the user with virtually unlimited information resources. To access these resources, you must use the appropriate application software. The user-friendly graphical interface of this software made Internet services available to everyone. Many of these programs run in the user's familiar Windows environment. Programs with a graphical interface have an important feature: they hide the entire system architecture from the user and allow you to work in the same way with information stored on computers of any platform.

A global computer network unites computers that are distant from each other over a long distance and which can be located in different cities, states and continents. The exchange of information between computers in such a network can be carried out using telephone lines, dedicated communication channels, including fiber optics, radio communication systems and satellite communications.

Global network structure

In general, a wide area network includes a communications subnetwork to which computers and terminals are connected (data input and display only). The global network may include local and regional networks as components (Fig. 15.1). The combination of global, regional and local computer networks makes it possible to create multi-network hierarchies. They provide powerful, cost-effective means of processing huge amounts of information and access to unlimited information resources. This is precisely the structure adopted in the most famous and popular now worldwide super-global information network, the Internet 1. The communication subnetwork consists of data transmission channels and communication nodes.

Rice. 15.1. Global network structure

Computers (usually personal) used by client users are called workstations. Computers that are sources of network resources provided to users are called servers. User workstations are connected to global networks most often through network access service providers - providers.

Communication nodes of the communication subnet are designed for fast transmission of information over the network, for choosing the optimal route for transmitting information and for switching packets of transmitted information. A communication node is either a hardware device or a computer that performs specified functions using appropriate software. These nodes ensure the efficient functioning of the communication network as a whole. The considered network structure is called a node structure and is used primarily in global networks.

Global Internet

About 20 years ago, the US Department of Defense created a network that was the progenitor of the Internet, it was called ARPAnet. ARPAnet was an experimental network; it was created to support scientific research in the military-industrial sphere, in particular, to study methods for constructing networks that are resistant to partial damage received, for example, during aircraft bombing and are capable of continuing normal functioning under such conditions. This requirement provides the key to understanding the principles of construction and structure of the Internet. In the model ARPAnet there was always a connection between the source computer and the destination computer (destination station). It was assumed that any part of the network could disappear at any moment.

Administrative device Internet

The Internet is an all-volunteer organization. It is governed by something like a council of elders, but the Internet does not have a president. The highest authority, wherever the Internet is, remains with ISOC (Internet Society). ISOC is a voluntary membership society. Its goal is to facilitate the global exchange of information via the Internet. It appoints a council of elders that is responsible for technical policy, support and management of the Internet.

The Council of Elders is a group of invited volunteers called IAB (Internet Architecture Council). The IAB meets regularly to approve standards and allocate resources, such as addresses.

It should be noted that there is no such organization that collects fees from all Internet networks or users. Instead, everyone pays their share. N.S.F. pays for maintenance NSFNET. NASA pays for the Scientific Network NASA (NASA Science Internet). Representatives of the networks get together and decide how to connect to each other and contain these relationships. A university or corporation pays for its connection to some regional network, which, in turn, pays a national network owner for its access.

Internet structure

The Internet is a collection of interconnected communication centers to which regional network service providers are connected and through which

their interaction takes place, i.e. The Internet has a structure typical of global networks (Fig. 15.1).

Until 1995, the Internet was controlled by the National Science Foundation (NSF), which created three powerful communications centers: in New York, Chicago and San Francisco. Centers were then established on the East and West Coasts and many other federal and commercial communications centers. Contractual relations are established between these centers on the transfer of information and maintenance of high-speed communications. The collection of communication centers forms a communications subnetwork supported by a number of powerful companies.

From the user's point of view, the service providers on the Internet are: providers(from English provider– “supplier”), maintaining information on servers and specializing in providing Internet access services, and consumers of these services – clients. The interaction of suppliers with consumers is carried out through a communication system with many nodes (Fig. 15.2).

Fig. 15.2. Logical diagram of the global Internet network

Principles of operation of the global network

The Internet is possible because standard methods of communication between computers and application programs have been developed. This allows computers of different types to communicate with each other without any problems. IAB responsible for standards; he decides when a standard is needed and what it should be. When a standard is required, the council considers the problem, adopts the standard, and broadcasts it to the world over the network. IAB also keeps track of various numbers (and other things) that must remain unique. For example, every computer on the Internet has its own unique 32-bit binary address. How is this address assigned? IAB cares about these kinds of problems. He does not assign addresses personally, but develops rules, rules for how to assign these addresses. The address is assigned by the specific provider that connects the computer to the network.

Let us consider in very general terms the principles of operation of a global packet-switching network using the TCP/IP protocol. This protocol underlies both the Internet and many others. Knowledge of the basics of network construction allows you to understand the meaning of many of the actions that the user will have to perform to gain access to numerous and varied network resources.

Network architecture

The network architecture is based on the multi-level principle of message transmission. At the lowest level, a message is a sequence of bits, accompanied by the address of the recipient and the sender. The message is divided into packets by network equipment and transmitted over communication channels. Added to this layer is a base software layer that controls the data communications hardware. The following levels of software are aimed at expanding the functionality of the network and creating a friendly, convenient and simple environment that provides user access to network resources and presentation of messages in a form familiar to the user.

The message is generated by the user at the highest level of the system. It sequentially passes through all levels of the system to the lowest, where it is transmitted via a communication channel to the recipient. As the message passes through each level of the system, it is provided with an additional header, which provides information to a similar level at the recipient node. At the recipient node, the message travels from the lower layer to the upper layer, stripping itself of headers. As a result, the recipient receives the message in its original form.

The standards provide for a seven-level network architecture model: Basic Reference Model for Open Systems Interconnection ( OSI). However, in practice, particularly on the Internet, the number of these levels is smaller.

Packet switching

A message (including a file) is transmitted over the network packages, which have a fixed length. The message is broken down into packets by the network adapter. Most adapters use packets between 500 and 4000 bytes in length. A data packet, similar to an envelope with a letter, has the address of the computer to which it is sent and the address of the computer that is sending the message. Obviously, the address of a computer on the network must be unique. On the receiving computer, the packets are compiled into a message.

When considering the operation of a network, natural associations arise with telephone communications. However, this is actually a misconception. Unlike the telephone network, it does not use circuit switching, in which some part of the network is allocated and blocked for direct communication between the transmitting and receiving nodes. The Internet is a packet-switched network and can be compared to the organization of regular mail. In postal services, all correspondence, regardless of where it is addressed, arrives at the post office. There it is sorted and further forwarded to various post offices with which there is communication, and which are not necessarily the final destinations, but bring the correspondence closer to its destination. The procedure is repeated at these post offices. The mail delivery service allows you to very accurately represent the procedure for transmitting packets over the network.

Routing

Delivery of packets in the network is carried out using communication nodes, which can be implemented in hardware or are programs on computers. These nodes connect individual computers and networks of different organizations and form a communications subnet. The main function of communication nodes is to select the optimal route delivery of the package to the recipient - routing. Each communication node does not have connections with all other communication nodes, and its function, like the function of the post office, is to determine the next node on the route that will best bring the package closer to its destination.

TCP/IP networks use 32-bit IP addresses to identify networks and computers. When written, these addresses are divided into 4 parts. Each 8-bit part can have a value from 0 to 255. The parts are separated from each other by dots. For example, 234.049.123.255.

An IP address includes the network number and the number of the computer on it. The addresses of each network are issued by the Internet Information Center ( NIC). A business must register with the NIC to obtain such an address before using the Internet. Even if you are not yet connected to the Internet, but are just about to connect, it is advisable to use IP addressing on your local network. The goal is to prepare the necessary address system.

As with postal mail, every packet sent over the network must have a recipient address and a sender address. At the communication node, the address of the packet recipient is checked and, based on it, the optimal path for sending the packet to its destination is determined. In each communication node, internal tables are built in which locations and all possible routes to all registered networks are recorded. The route includes all communication nodes on the way to the destination. Using these tables, the router calculates the shortest path to the destination, and if there is a failure in the route, it looks for another path.

The package and the addresses indicated on it must be issued according to certain rules. These rules are called protocol. The IP (Internet Protocol) protocol, responsible for addressing, ensures that the communication node determines the best route for delivering the packet.

Internet addressing

When exchanging data on a network, it is necessary that each computer has its own unique address. In a local network, computer addresses are most often determined by the addresses of the network cards inserted into the computers. Network cards (Ethernet) have unique addresses that are set during their manufacture. In addition, it is possible to enter addresses that are more convenient for a given organization when configuring the board. The host address is a 12-digit hexadecimal number. Each LAN segment also has a network address. This addressing is used on a NetWare network.

IP addresses are used when sending and receiving messages over the TCP/IP protocol. However, it is inconvenient for the user to use such addresses when organizing communication with another computer on the network to receive some service. Therefore, the Domain Name System (DNS) was introduced into the Internet. In this system, computers on the network are given user-friendly names, behind which the corresponding addresses are hidden.

Domain name system

Networks and computers connected to the Internet have unique symbolic identifiers called domain names. These unique names, as well as network addresses, are registered with the NIC and stored in the Internet database.

A domain name consists of two parts: a business identifier and a domain identifier (top-level domain), separated by a dot. For example, com– domain identifier, which is a standard for identifying commercial organizations. Domain ID edu is standard for educational organizations. There are six standard domain identifiers registered with the NIC - two named ( com And edu), and gov(government organizations), mil (military organizations), org(non-profit organizations), net(network organizations). These domain identifiers are mainly used by US organizations.

In other countries, the two-letter country in which the organization is located is used as domain identifiers. There are identifiers for all countries of the world. Identifiers are valid for our country ru And su.

Network names below the root domain ( com, edu, su etc.) are enterprise identifiers and must be registered in the NIC network information center to ensure their uniqueness. An enterprise that has a primary domain is responsible for administering its address space and determines the names located to the left of the organization's name in the domain name.

Network domain addresses contain a sequence of names separated by dots. Moreover, the clarification of which computer the address belongs to is made from right to left. For example, nvp.finec.ru means that the computer is located in Russia (ru), at the University of Economics and Finance (finec), and in the university network it has the name nvp.

On the Internet, the Domain Name System (DNS) handles the translation of names into addresses. Essentially, it is a database that records the correspondence between domain names and IP addresses. This system allows you to use domain names instead of IP addresses. The TCP/IP protocol works with IP addresses and cannot (by itself) use domain addresses. The communications node (gateway) must know the addresses of several DNS servers in order to resolve user-entered names into equivalent IP addresses. If the DNS name server does not have name information, it returns the IP address of another (capable of answering the query) DNS name server.

IP addresses are assigned to a computer from a set of IP addresses reserved for the organization. In this case, the IP address of the gateway to which the message must be sent that does not have a destination address is also indicated. Registration of a domain name, assignment of an IP address, and provision of access to network services may be the responsibility of the provider.

Internet transmission control

Transmission control is implemented by the TCP (Transmission Control Protocol), which breaks the transmitted message into packets and assembles the received message from the packets. The TCP protocol monitors the integrity of the transmitted packet and controls the delivery of all message packets. Thus, on the Internet at the internetwork level, the IP protocol provides non-guaranteed data delivery between any two network points, and the TCP transmission control protocol, being a superstructure over the IP protocol, ensures guaranteed data delivery.

These protocols, by defining the formats of data packets transmitted over the network, allow programs running on different hardware and software platforms to exchange information.

TCP/IP protocol is not limited to the lower-level protocols IP and TCP included in it. Being a family of protocols (more than a dozen) used in both global and local networks, TCP/IP defines the rules for the operation of other network layers.

FTP-protocol, part of the TCP/IP protocol family, is a user-level protocol that allows file transfer from one computer to another. This protocol allows you to send files in various formats, most often text or binary, without loading the remote computer's CPU, since it does not involve running sessions on the remote computer.

Telnet protocol belongs to the same group of protocols as FTP, but is a remote terminal access protocol that allows one computer to connect to another and work on it, as if working directly on a computer. Thus, Telnet allows you to connect to a host computer, log on to it, and run programs on it.

SMTP protocol(Simple Mail Transfer Protocol) enables the transfer of email between computers.

SNMP protocol(Simple Network Management Protocol) transmits information about the state of the network and devices connected to it.

The TCP/IP protocol has well-defined specifications and is supported by many manufacturers of both hardware and software, ensuring compatibility, and is the most popular protocol in the world.

Internet connection methods

Connecting an individual computer

To connect an individual computer to the Internet, it is enough to have a modem, a telephone line and an organization that has a gateway to the Internet. Numerous providers offer dial-up ( dial-up) access of an individual computer with a modem via telephone lines. In this case, it is possible to use the supplier’s computer directly connected to the Internet to gain access to Internet resources. Such a computer is called host (leading computer, or host machine). On the host, the user runs client programs available from the supplier and available to him, which allow him to gain access to the desired server and its information.

Modem is a device that is simultaneously connected to a computer and a telephone line. It receives digital information from a computer and turns it into an analog signal suitable for transmission over a telephone line ( modulation). In addition, it is capable of receiving a modulated signal from another modem, converting it into digital form and transmitting it to its computer ( demodulation).

Hence the name MODEM - MOdulator-DEMOdulator.

In addition, the modem can interact with the switched telephone network - dial a number and recognize free and busy signals. Modems perform a number of other functions, the most important of which are error correction and information compression.

Direct connection to the Internet of the organization's local network

Direct ( on- line) connection to the Internet of the organization's local network is carried out via dedicated leased communication lines using additional software. Typically used by organizations that connect a large number of computers connected to a local network. To access Web servers and other Internet resources, each user must have an IP address.

The NetWare LAN connects to the Internet through a gateway. The gateway provides access to the Internet for every network user. The user can run all programs receiving Internet services from the standard NetWare client environment. Moreover, most of the work can be performed in the Windows environment (Fig. 15.3).

Rice. 15.3. Direct connection to the Internet local network

organizations

Internet services

The Internet service is built on the client-server model. A server is a program that supports a specific network service. Users of other Internet nodes have access to this service through a client program. Most client programs provide the user with a graphical interface that makes access to the service simple and convenient. The service server allows you to organize information in a standard form, as well as receive client requests, process them and send a response to the client.

Let's look at the most well-known services provided by servers on the global Internet.

Email

One of the means of interaction between users in networks is electronic mail (e-mail). The creation of the Internet began with e-mail and it remains the most popular type of activity on it.

In general, email is a broad term used to describe the process of transmitting messages between computers. There are emails used in local and global networks. Next we will talk about global email systems.

The advantages of e-mail include: speed and reliability of correspondence delivery; relatively low cost of services; the ability to quickly familiarize a wide range of correspondents with the message; sending not only text messages, but also programs, graphic images, audio files; saving paper, etc.

General principles of operation of e-mail systems

Let's look at the basic diagram underlying the operation of various email systems.

To send an email message using your computer, you call your email program, specify the recipient of the message, create the message itself, and instruct the program to send it. A signal to transmit a message establishes a connection between your computer and an email host computer directly connected to one or another global network. The message, reaching the sender's host computer, is then transmitted via communication channels to the recipient's machine and there it is placed in a disk memory area owned by the recipient and called a mailbox. The recipient user takes incoming mail from the mailbox to his computer and processes it.

Any email system consists of two main subsystems:

1) client software with which the user directly interacts;

2) server software that controls the reception of a message from the sending user, the transmission of the message, the sending of the message to the recipient's mailbox and its storage in this mailbox until the recipient user takes it from there.

Different email programs can be classified according to different criteria. For example, what operating system they can run on. Nowadays, the most widespread products are those running on the Windows OS. Mail processing programs included in the Microsoft Internet Explorer and Netscape Navigator browsers are widely used. A browser (from the English browser) is a program that searches the Internet. (For more information about browsers, see below in “World Wide Web WWW”). There are programs for users UNIX systems and OS/2.

Required for email to work special programs. There are two main e-mail standards:

SMTP (Simple Mail Transfer Protocol);

SMTP standard It is attractive due to its simplicity, low cost, and many service functions and, as a result, has become widespread, in particular, on the Internet. There is also the POP-3 standard, which differs from SMTP mainly in that in this standard the client works with a program installed on the provider's computer, and not on his own computer.

X.400 standard It is distinguished by rigor, strict standardization, the presence of commercial operators with a guaranteed level of service, and support for a large number of national codes. Due to these features, this standard is very popular among government organizations around the world when working, in particular, on government telecommunication lines.

Among the many e-mail programs running under Windows in the SMTP standard, we can name, for example:

Outlook Express, used in the MS Internet Explorer browser;

Netscape Mail, part of the Netscape Navigator browser;

Mail, HotMail, Hotbox and others free programs in Internet;

MSMail, included in office application Outlook;

Qualcomm's Eudora Pro and many others.

Despite the variety of client programs for various email systems, they all have common functions:

notification of the arrival of new mail;

reading incoming mail;

creating outgoing mail;

message addressing;

using an address book containing a list of subscribers to whom mail is often sent;

sending messages;

message processing and storage. Message processing includes functions such as printing, deleting, forwarding messages, sorting, archiving messages, storing related posts. Particular attention should be paid to programs that allow you to work with folders and create your own folders for storing messages on various topics. This is very convenient and helps you process mail faster and more efficiently.

Working with attached files. Using the file attachment capabilities of email messages, you can send any binary file via email.

Mail message structure

Any message consists of a header and the message body itself (Fig. 15.4).

Rice. 15.4. Mail message structure

Heading includes: email recipient address (field That); your return address (field From); email subject (field Subject; it should be short and informative); date and time of sending the letter (field Date); recipients who will receive a copy of the letter (fields Ss And Vss, the differences between these fields are that the recipients listed in the field Vss, will not appear in the email header in the recipients field, this field is called the blind carbon copy field); a list of files sent along with the letter.

The email address generally looks like this:

[email protected]

The address consists of two parts: the username and the address of the mail host computer on which the user is registered. The two parts of the address are separated by the @ sign.

The specific address of the subscriber may look, for example, like this: [email protected]. The part of the address to the right of the @ sign means: ru – Russia, uef – St. Petersburg University of Economics and Finance, main is the name of the host computer on which the user lina is registered (or a mailbox with that name is installed).

The header is separated from the message text by a blank line. At the end of the text there may be signature – electronic signature, but this is optional.

After reading the mail, you can: reply to the letter, redirect (the recipient will receive it on behalf of the original sender) or forward it to another recipient with your comments, print, save and, finally, delete.

Mail on the user's computer is stored in folders. Folders are divided into those built into the package and those created by the user. Built-in folders include incoming mail folders ( In), outgoing mail ( Out) and garbage ( Trash). A folder is accessed by clicking on its name in the menu Mailbox. You can open several folders at the same time. The window of any folder contains the following information about the messages included in it: status/priority, sender/recipient, date, size, subject. You can create your own folders to complement the built-in ones. The user himself determines which folders are convenient for him to have.

Transferring files

If you find the information you need online, it's often best to work with a copy of it on your computer. To obtain a copy of the file, the FTP program is used, which gets its name from the corresponding protocol - File Transfer Protocol.

The FTP program is part of the standard set of application level programs of the TCP/IP protocol family and is designed for transferring files between computers. It allows you to access FTP servers connected to the Internet and containing files that can be retrieved by any user.

Working with the FTP program is simple. By running the program on your computer, you can give the command OPEN - open the server. Next, you can view the contents of the directories and use the GET command to get the file to your computer. HELP will help you learn about the purpose of other commands. Work with FTP servers can take place in real time. It is possible to receive files from FTP servers and via Internet e-mail. Anonymous access to numerous open databases, implemented by a special FTP service program, is widespread. Due to this, you can receive files without presenting your name and password. To receive a file in the FTP system, indicate: the exact name of the node, the name of the directory, subdirectory, file name.

Receiving network services through a remote computer

Telnet, a protocol for remote terminal access to the network, allows you to receive Internet services using the resources of a remote computer. Telnet connects your computer to a remote computer connected to the Internet, and you can work on your computer as if you were sitting at a terminal on the remote system. All commands entered on your computer are executed by the remote computer system.

Working on a remote computer using Telnet, you can run any client programs available on it that will allow you to receive the desired service. Telnet can also transfer files, but FTP is more efficient and uses less CPU. The Telnet program has many versions.

Teleconferences

Systems that allow you to read and send messages to open information groups, called electronic bulletin boards or newsgroups, are very popular on the Internet. These systems are designed to facilitate discussions and exchange of news. The largest teleconferencing system in the world is USENET NEWS. It has groups - teleconferences on a wide variety of topics. A user can subscribe to any of these topics to take part in a discussion on the topic of that conference or view the news.

If you have direct access to the Internet, work in the teleconferencing system begins by entering the program name news at the command line. Through the displayed menus, you can get a list of groups available to you on the specified news server, select the desired group and simply click subscribe to it. Having opened the group, you can view the news, take part in the discussion by sending your message to the group.

To make it easier for the user to navigate a huge number of groups, the group names use abbreviations accepted by the system. Groups can be selected using a set of keywords you specify. Access to teleconferences can be made not only in on-line mode. Teleconferences can also be accessed via email. Of course, you will receive news only after some time.

The procedure for filling out conferences is provided by the participants themselves. Therefore, there are rules of conduct that may vary from conference to conference, for example:

news.answers- rules of world conferences, in English

relcom.answers- teleconference rules in Russian

USENET NEWS can be accessed in a variety of ways. The most convenient and correct way is to use special reading programs, for example, nn or tin. This method is usually used by users of Unix systems. These programs have a fairly long history, have advanced capabilities, and are preferred by experienced users. However, for beginners, we can recommend the tin program, if it is available and configured.

Facilities mobile communications and Internet

The development trend of modern communication technologies eloquently suggests that in the next few years a new section will appear in the communication services market - mobile Internet or Internet using mobile communications.

Now in St. Petersburg the standard is used WAP(Wireless Application Protocol), which today is the basis for data transmission through cellular operators. In addition, the standard is checked in test mode GPRS(General Packet Radio Service). The difference between these protocols is that the first uses a dedicated channel to transmit information, while the latter uses packets when transmitting data that can be transmitted without using a dedicated channel, which significantly increases the throughput of the transmitting equipment.

In order to provide Internet information to mobile phone users, it must be created using the language WML(Wireless Markup Language). In this case, we are not talking about using a mobile phone as a switching device, in other words, a modem, but about using it as a means of viewing information.

There are now a sufficient number of resources that can be used in this area. For example , http://www.nevru.com/wap/index.shtml. Information provided for mobile phones can also be viewed using standard browsers. To do this, you need to enter, for example, http://wapsilon.com/ in the address bar - a special server for viewing WAP resources, and then in the window that opens, enter the desired resource in the input line, for example, wap. rosweb. ru. In addition, mobile phones allow you to transmit information using short SMS text messages. The limitation for SMS messages is their size - 160 characters in one message; moreover, if the message is written in Russian, the message is reduced to 80 characters.

Interactive communication between users in natural language

Interactive communication between users in natural language or teleconferences in real time is implemented by the IRC (Internet Relay Chat) system. This system is designed for live conversations and exists thanks to the high speed of information transfer on the Internet.

A group of users can communicate in real time at once. IRC servers provide support for communication on a variety of topics. Typically, each group united by a topic communicates almost continuously (in the sense that the response delay time is extremely short). Some people stop communicating, new ones come and get involved in the conversation. When working with this program, the user on one part of the screen sees constantly incoming information on the selected topic, and on the other part he can place his messages in the same group, which are immediately sent to the displays of all other participants in this group.

To connect to IRC, you must have the appropriate client program and type its name on the command line to start it. The program will automatically connect you to one of the IRC servers. Since all IRC servers are connected into a single world space, contacting one of them puts you in that space.

World Wide Web WWW

WWW 1 (World Wide Web) is an attempt to combine the capabilities of all these means in one information tool, and even add to them the transmission (in addition to texts and programs) of graphic images, sounds, and videos. All these information objects are connected by the hypertext structure.

Hypertext is a system of documents with cross-references, i.e. pointers from one document to another. Since the WWW system allows these documents to include not only texts, but also graphics, sound and video, a hypertext document has become a hypermedia document. Documents contain links to other documents that are related in meaning, for example, that deepen the understanding of a given text. Pictures, sound clips, and video clips can be associated with links. Pictures or parts of them may also include links to text, new pictures or sound. Documents referenced may be located on remote computers. Using links you can move significantly away from the original source of information, but you can easily return to it. Thus, when reading an article about an art gallery, you can immediately view its paintings, and when studying musical instruments, you can hear their sound.

Hypermedia documents are stored on WWW servers on the Internet. To work with hypermedia documents, many different client programs have been developed, called WWW viewers, or browsers 2 . Viewing programs allow you to call up the documents you need at a known exact address, accumulate them, sort them, combine them, edit them, and print them.

The most popular browsing programs are Microsoft Internet Explorer and Netscape Navigator. These browsers have a lot in common. Therefore, having mastered one of them, it is easy to switch to working with another. If you do not know the exact address of the document you are interested in, you need to contact search servers.

Search servers can be classified according to the principle of information presentation:

search engines,

yellow Pages,

When using WWW technology, resource developers can set keywords in the service information section. For example, for the website of a university of economics and finance, the keywords could be: education, training, university, etc.

Search engines read these keywords and write them to their database. When searching for the required keyword, the searched information is compared with the database and with information on the Internet, after which the user is provided with a list of search results. The list is created based on the principle of the most suitable answer to the query.

To search for information on the WWW, there are international search engines (search programs) AltaVista, Lycos, Yahoo, etc. For Russian-language searches, domestic search engines Rambler, Yandex and Aport are more convenient. When working with search engines, the user sets search image- keywords of the topic of interest, and the system provides lists and addresses of those documents in which these words appear. Note that although there are many good search programs available, it is best to have an exact address. The method for specifying the address is determined by the system of unified URLs(URL = Uniform Resource Locator - unified resource locator).

The search program accesses search servers accessible via the Web interface to select the desired addresses. The main function of these servers is to process information from documents on various servers (Web, FTP, Usenet, etc.), enter it into a database and provide addresses of this information at the request of search program users.

To search servers “ yellow Pages” refer to servers that not only search for information of interest, but also store in their databases the phone, fax, regular and email addresses of the organization.

An example would be:

www. yellow. com

An example would be:

www. rmp. ru

Global network - These are associations of computers located at a remote distance for the common use of the world's information resources. Today there are more than 200 of them in the world. Of these, the most famous and most popular is the Internet.

Unlike local networks in global networks there is no single control center. The network is based on tens and hundreds of thousands of computers connected by one or another communication channels. Each computer has a unique identifier, which allows you to “plot a route to it” for the delivery of information. Typically, a global network unites computers that operate according to different rules (having different architectures, system software, etc.). Therefore, gateways are used to transfer information from one type of network to another.

Gateways are devices (computers) used to connect networks with completely different exchange protocols.

Exchange protocol is a set of rules (agreement, standard) that defines the principles of data exchange between different computers on the network.

Protocols are conventionally divided into basic (lower level) responsible for transmitting information of any type, and applied (higher level) responsible for the functioning of specialized services.

The host computer of a network that provides access to a common database, enables sharing of input/output devices and user interaction is called server.

A network computer that only uses network resources, but does not give its resources to the network itself is called client(often also called workstation).

To work on the global network, the user must have the appropriate hardware and software.

Software can be divided into two classes:

server programs that are located on the network node serving the user’s computer;

client programs located on the user's computer and using the services of the server.

Global networks provide users with a variety of services: email, remote access to any computer on the network, search for data and programs, etc.

Hardware implementation of LAN

In the simplest case, network cards and a cable are enough to operate the network. If you need to create a fairly complex network, you will need special network equipment.

Computers within a local network are connected using cables that transmit signals. A cable connecting two network components (for example, two computers) is called a segment. Cables are classified depending on the possible values ​​of information transfer speed and the frequency of failures and errors. There are three main categories of cables most commonly used:

unshielded twisted pair . The maximum distance at which computers connected by this cable can be located reaches 90 m. Information transfer speed is from 10 to 155 Mbit/s; shielded twisted pair. Information transfer speed is 16 Mbit/s over a distance of up to 300 m.

To oaxial cable . It is characterized by higher mechanical strength, noise immunity and allows you to transmit information over a distance of up to 2000 m at a speed of 2-44 Mbit/s;

fiber optic cable . An ideal transmission medium, it is not affected by electromagnetic fields, allows you to transmit information over a distance of up to 10,000 m at a speed of up to 10 Gbit/s.

Twisted pair cables are now most widely used to build local networks. Inside, such a cable consists of two or four pairs of copper wire twisted together. Twisted pair also has its own varieties: UTP (Unshielded Twisted Pair) and STP (Shielded Twisted Pair). These types of cable are capable of transmitting signals over a distance of about 100 m. As a rule, UTP is used in local networks. STP has a braided copper filament jacket, which has a higher level of protection and quality than the jacket of a UTP cable. In an STP cable, each pair of wires is additionally shielded (it is wrapped in a layer of foil), which protects the data that is transmitted from external interference. This solution allows you to maintain high transmission speeds over longer distances than when using UTP cable. The twisted pair cable is connected to the computer using an RJ-45 (Registered Jack 45) connector, which is very similar to an RJ-11 (Registered Jack 11) telephone connector.

Twisted pair cable is capable of providing network operation at speeds of 10, 100 and 1000 Mbit/s.

Coaxial cable consists of a copper wire covered with insulation, a metal braided shield, and an outer sheath. The central wire of the cable transmits signals into which the data has previously been converted. Such a wire can be either solid or multi-core. To organize a local network, two types of coaxial cable are used: ThinNet. (thin, 10Base2) and ThickNet (thick, 10Base5). At the moment, local networks based on coaxial cable are practically not found. The information transmission speed in such a network does not exceed 10 Mbit/s. Both types of cable, ThinNet and ThickNet, connect to a BNC connector, and terminators must be installed at both ends of the cable.

At the heart of a fiber optic cable are optical fibers (light guides), through which data is transmitted in the form of pulses of light. No electrical signals are transmitted through a fiber optic cable, so the signal cannot be intercepted, which virtually eliminates unauthorized access to data. Fiber optic cable is used to transport large amounts of information at the highest available speeds. The main disadvantage of such a cable is its fragility: it is easy to damage, and can only be mounted and connected using special equipment,

2. Network cards

Network cards make it possible to connect a computer and a network cable. The network card converts the information that is intended to be sent into special packets. A packet is a logical collection of data that includes a header with address information and information itself. The header contains address fields, which contain information about the origin and destination of the data. The network card analyzes the destination address of the received packet and determines whether the packet was actually destined for this computer. If the output is positive, the board will transmit the packet to the operating system. Otherwise, the package will not be processed. Special software allows you to process all packets that pass within the network. This opportunity is used by system administrators when analyzing the operation of the network, and by attackers to steal data passing through it. Any network card has an individual address built into its chips. This address is called a physical or MAC address (Media Access Control). The order of actions performed by the network card is as follows. Receiving information from the operating system and converting it into electrical signals for further sending over the cable. Receiving electrical signals over a cable and converting them back into data that can be processed operating system. Determining whether the received data packet is intended specifically for this computer. Controlling the flow of information that passes between a computer and a network.

More often network cards integrated into motherboard and connect to the southbridge. The processor communicates with the south bridge, and all equipment that is connected to it, through the north bridge.

In addition, the operating system of each computer connected to the network must have network support tools: special system and user programs, as well as a set of specific rules governing the forms and procedures for exchanging information over the network between two or more devices (or processes), which are called network protocols

3. Repeaters

The local network can be expanded through the use of a special device called a “repeater” (Repeater). Its main function is to, having received data on one of the ports, redirect it to the remaining ports. These ports can be of any type: RJ-45 or Fiber-Optic. Combinations also do not play a role, which allows you to combine network elements that are built on the basis various types cable. Information during transmission to other ports is restored to eliminate deviations that may appear during the movement of the signal from the source.

Repeaters can perform a separation function. If the repeater determines that there are too many collisions on one of the ports, it concludes that there is a problem on that segment and isolates it. This feature prevents failures in one segment from spreading to the entire network.

The repeater allows you to:

§ connect two network segments with the same or different types of cable;

§ regenerate the signal to increase the maximum transmission distance;

§ transmit data flow in both directions.

4. Hubs

A hub is a device that can connect computers into a physical star topology. The hub has several ports that allow you to connect network components. A hub with only two ports is called a bridge. A bridge is required to connect two network elements.

The network together with the hub constitutes a “common bus”. Data packets when transmitted through the hub will be delivered to all computers connected to the local network.

There are two types of hubs:

§ Passive concentrators. Such devices send the received signal without preprocessing it.

§ Active hubs (multiport repeaters). They receive incoming signals, process them and transmit them to connected computers.

5. Switches

Switches are needed to organize closer network connection between the sending computer and the destination computer. During data transfer through the switch, information about the MAC addresses of computers is recorded in its memory. Using this information, the switch compiles a routing table, in which for each computer it is indicated that it belongs to a specific network segment.

When a switch receives data packets, it creates a special internal connection (segment) between its two ports using a routing table. It then sends a data packet to the appropriate port on the destination computer, based on the information described in the packet header.

Thus, this connection is isolated from other ports, which allows computers to exchange information at the maximum speed that is available for this network. If a switch has only two ports, it is called a bridge.

The switch provides the following features:

§ send a packet with data from one computer to the destination computer;

§ increase data transfer speed.

6. Routers

A router is similar in principle to a switch, but has a greater range of functionality. It studies not only the MAC, but also the IP addresses of both computers involved in the data transfer. When transporting information between different network segments, routers analyze the packet header and try to calculate the optimal path for the packet to travel. The router is able to determine the path to an arbitrary network segment using information from the route table, which allows you to create a shared connection to the Internet or WAN.

Routers allow packets to be delivered in the fastest way, which increases throughput large networks. If some segment of the network is overloaded, the data flow will take a different path.

An ordinary computer can be used as a simple router.

With the advent of the first personal computers their development was closely followed by millions of people around the world. Progress was very rapid and steady. Computer technology has entered and forever strengthened in all spheres of society.

Now it is no longer possible to imagine the isolation of any human activity from computer devices. Users spend their leisure time online, the work of millions of offices of commercial and non-profit companies is ensured by computers, the defense and security of entire countries depend on the development of this type of technology.

To achieve all the tasks and goals that the creators of computer devices set for themselves, it was necessary to find a way to connect user stations to each other.

Having gone through a difficult path of development, such methods were found that allow connecting several stations.

1. Local networks– allow you to connect stations at a distance of no more than 15 km. In this way, most often, either a single building or several neighboring ones are connected.


2. Regional networks– the length of which ranges from 10 to 100 km. A regional network can be either a city or an entire district.


3. Global networks– provide connections over 1000 km. They unite regions and even countries. The most famous such network, of course, is the Internet.

There is no need to consider regional networks in this article, since a lot will be said about them at the Svyaz Expocenter exhibition, and information about what they are will be presented here.

Local computing network

LANs are very popular in economic sphere, allowing you to combine office workers' stations.

Different connection patterns such as busbar, ring and star are used to achieve the same purpose. The bus represents a scheme in which there is a single central channel, and all users are connected to it. Communication is carried out between all computers on the network.

This scheme pleases with its low connection cost and reliability. If one of the network stations malfunctions, this will not in any way affect the performance of the others.

The bus also has a number of disadvantages, including low data transfer speed and its dependence on the number of network subscribers, as well as extremely low security of information stored on the hard drives of user stations.

When connecting according to the “ring” scheme, stations are literally connected sequentially in a circle, forming a closed ring. Information during the transmission of packet data contacts all users who are connected by the network until it reaches the user with the desired address.

The advantages here are security, speed of data exchange and significant length of connections. Subscribers of such a network are very vulnerable - if one station fails, the communication of the others is lost.

The latest and most popular local network scheme today is the “star”. Stations are connected to a central node (server or hub) and exchange information through it. The advantages here are high speed, separate communication channels and the fact that a malfunction of any station does not affect the performance of the network.

Local and global computer networks is a very important topic at the Communications exhibition at the Expocenter, so the possibilities and advantages of giving client-servers special rights will be discussed there, connection details will be considered, as well as prospects for the development of these networks.

Global Area Networks

Wide Area Networks (WAN) are a way to connect many computers around the world. The connection of a huge number of local and regional networks between many countries made it possible to create the Internet using a colossal number of data transmission channels.

In addition to the most common network, there are others, among which are FidoNet, Eunet, Gren. There are also corporate networks, they are created by individual companies in order to protect their own information stored on networked computers of office employees in different countries.

Today, the ability to connect and use local and global computer networks is fundamental when choosing computerized devices for buyers. All gadgets, by modern standards, must provide users with Internet access.

At the exhibition "Communication", which will be held by Expocentre, we will talk about the global significance that the Internet has acquired from its creation to the present day. This is not the first time for this company to host an event touching on a global topic. The Expo Center annually hosts exhibitions in key areas of society.

Local and global computer networks, undoubtedly deserve attention at the event held by the Expocenter, which will be interesting and informative.

Read our other articles:

A lot has been said. This is not surprising, given the fact that currently a huge number of electronic devices provide the ability to connect to it. These are not only computers, but also ATMs, smart home systems, communication devices and even televisions. In general, it is fundamentally impossible to provide the entire list. In fact, if telephone networks allow people to communicate, then the global Internet makes it possible for electronic devices to exchange information.

It is necessary to distinguish between local networks, consisting of several computers in a relatively small area, and the Internet, which covers the whole world.

The history of the Internet began back in nineteen fifty-seven, when the American Department of Defense, concerned about the problem of communication in a war with a possible enemy, invited the country's four leading universities to develop a network for the exchange of digital data between electronic computing devices. The result of their activities was the ARPANET network, which appeared in September 1969 and connected these universities.

On October twenty-ninth of the same year, the first attempt at computer communication was made between nodes in California and Stanford (six hundred and forty kilometers). At 22:30 a stable connection was established, and this is what is considered the birth of the Internet (although, in fact, it was still the ARPANET network).

Then various programs for exchanging e-mail begin to appear and are actively used. At the same time, the concept of “mailing” first appeared. Although ARPANET was the largest, in parallel with it there were computer networks, the work of which was carried out on the basis of other technical and software solutions. It was obvious that some standard was needed that would allow them to interact with each other. So, since January 1983, ARPANET switched to using the TCP/IP protocol (instead of NCP). It is believed that it was from this moment that the global Internet began its victorious march across the Earth.

In 1984, the one that is still in use was introduced. In the same year, another large network appeared - NSFNet (American Science Foundation). Its peculiarity is that it consisted of several small networks, therefore it was more flexible in scaling than ARPANET. So, in just a year the number of connected machines exceeded 10 thousand, which was quite a lot at that time. After this, the term “global Internet” began to be used specifically for NSFNet.

In 1988, the IRC real-time protocol appeared, providing the ability to organize chats.

A year later, the HTML language and the corresponding protocol appeared, serving as the beginning of the creation of the World Wide Web.

In 1990, ARPANET disappeared, finally losing to NSFNet. Since nineteen ninety-one, all data on the World Wide Web has become accessible via the Internet. And after the creation of the Mosaic browser in 1993, the global Internet became more popular and more accessible every year.

General principles technical implementation is as follows: a certain provider company provides end users (computers) with access to information. All computers are connected to the servers of this company, and from there to the requested addresses on the global network. Moreover, the address itself can be either the host server itself (on which the resource is located) or the end computer. In other words, there is a system of branches similar to blood vessels or even neural connections in the brain.

Now the development of the Internet is aimed at increasing the introduction of the next version of the IP protocol and optimizing existing operating principles.

» [Informatics Exam][Ticket No. 22]

Local and global computer networks. Addressing in networks.

A computer network is a collection of computers and various devices, providing information exchange between computers on a network without the use of any intermediate storage media.

The creation of computer networks is caused by the practical need of users of computers remote from each other for the same information. Networks provide users with the opportunity not only to quickly exchange information, but also to collaborate on printers and other peripheral devices, and even to process documents simultaneously.

The entire variety of computer networks can be classified according to a group of characteristics:

• Territorial distribution;
• Departmental affiliation;
• Information transfer speed;
• Type of transmission medium;

According to the territorial distribution, networks can be local, global, and regional.

By affiliation, departmental and state networks are distinguished. Departmental ones belong to one organization and are located on its territory.

Based on the speed of information transfer, computer networks are divided into low-, medium- and high-speed.

Based on the type of transmission medium, they are divided into coaxial networks, twisted pair networks, fiber optic networks, with information transmission via radio channels, and in the infrared range.

Local computer networks.

A local network unites computers installed in one room (for example, a school computer lab consisting of 8-12 computers) or in one building (for example, in a school building several dozen computers installed in different subject rooms can be combined into a local network).

In small local networks, all computers usually have equal rights, i.e., users independently decide which resources of their computer (disks, directories, files) to make publicly available over the network. Such networks are called peer-to-peer.

If more than ten computers are connected to the local network, then the peer-to-peer network may not be efficient enough. To increase productivity, as well as to ensure greater reliability when storing information on the network, some computers are specifically dedicated to storing files or application programs. Such computers are called servers, and a local area network is called a server-based network.
Each computer connected to the local network must have a special card (network adapter). Computers (network adapters) are connected to each other using cables.

Network topology.

The general scheme of connecting computers into local networks is called network topology. Network topologies can be different.

Ethernet networks can have a bus or star topology. In the first case, all computers are connected to one common cable (bus), in the second, there is a special central device (hub), from which “rays” go to each computer, i.e. Each computer is connected to its own cable.

The bus type structure is simpler and more economical since it does not require additional device and uses less cable. But it is very sensitive to cable system faults. If the cable is damaged in even one place, then problems arise for the entire network. The location of the fault is difficult to locate.

In this sense, the “star” is more stable. A damaged cable is a problem for one specific computer; it does not affect the operation of the network as a whole. No effort is required to locate the fault.

In a network with a “ring” type structure, information is transmitted between stations along the ring with re-reception in each network controller. Reception is carried out through buffer drives made on the basis of random access memory devices, so if one network controller fails, the operation of the entire ring may be disrupted.
The advantage of the ring structure is the ease of implementation of devices, and the disadvantage is low reliability.

Regional computer networks.

Local networks do not allow shared access to information among users located, for example, in different parts of the city. Regional networks come to the rescue, connecting computers within one region (city, country, continent).

Corporate computer networks.

Many organizations interested in protecting information from unauthorized access (for example, military, banking, etc.) create their own, so-called corporate networks. A corporate network can connect thousands and tens of thousands of computers located in various countries and cities (an example is the Microsoft Corporation network, MSN).

Global computer network Internet.

In 1969, the ARPAnet computer network was created in the United States, uniting computer centers of the Department of Defense and a number of academic organizations. This network was intended for a narrow purpose: mainly to study how to maintain communications in the event of a nuclear attack and to help researchers share information. As this network grew, many other networks were created and developed. Even before the advent of the personal computer era, the creators of ARPAnet began developing the Internetting Project program. The success of this project led to the following results. First, the largest Internet network in the United States (with a lowercase i) was created. Secondly, various options for interaction of this network with a number of other US networks were tested. This created the preconditions for the successful integration of many networks into a single global network. Such a “network of networks” is now called the Internet everywhere (the Russian spelling Internet is also widely used in domestic publications).

Currently, tens of millions of computers connected to the Internet store a huge amount of information (hundreds of millions of files, documents, etc.) and hundreds of millions of people use the information services of the global network.

The Internet is a global computer network that unites many local, regional and corporate networks and includes tens of millions of computers.

Each local or corporate network usually has at least one computer that has a permanent connection to the Internet using a high-bandwidth link (Internet server).

The reliability of the global network is ensured by the redundancy of communication lines: as a rule, servers have more than two communication lines connecting them to the Internet.

The basis, the “framework” of the Internet consists of more than one hundred million servers constantly connected to the network.

Hundreds of millions of network users can connect to Internet servers using local networks or dial-up telephone lines.

Internet addressing

In order to contact a certain computer on the Internet, you need to know its unique Internet address. There are two equivalent address formats that differ only in their form: IP address and DNS address.

IP address

An IP address consists of four blocks of numbers separated by periods. It may look like this:
84.42.63.1

Each block can contain a number from 0 to 255. Thanks to this organization, over four billion possible addresses can be obtained. But since some addresses are reserved for special purposes, and blocks are configured depending on the type of network, the actual number of possible addresses is slightly less. And yet, it is more than enough for the future expansion of the Internet.

The concept of “host” is closely related to the concept of IP address. A host is any device that uses the TCP/IP protocol to communicate with other equipment. This can be not only a computer, but also a router, hub, etc. All these devices connected to the network must have their own unique IP address.

DNS address

The IP address is numeric, as computers use it in their work. But it is very difficult to remember, so a domain name system was developed: DNS. The DNS address includes more user-friendly letter abbreviations, which are also separated by dots into separate information blocks (domains). For example:

If you enter a DNS address, it is first sent to a so-called name server, which converts it into a machine-readable 32-bit IP address.

Domain names

A DNS address usually has three components (although there can be any number of them).

The domain name system has a hierarchical structure: top-level domains - second-level domains, and so on. Top-level domains are of two types: geographical (two-letter - each country has its own code) and administrative (three-letter).

Russia owns the geographic domain ru.

The [email protected] portal has registered the second-level domain klyaksa in the top-level administrative domain net.

The names of computers that are Internet servers include the fully qualified domain name and the actual computer name. So the full address of the [email protected] portal looks like www.site

gov - government agency or organization
mil - military establishment
com - commercial organization
net - network organization
org - an organization that does not belong to one of the above

Among the frequently used country identifier domains are the following:

at - Austria
au - Australia
ca - Canada
ch - Switzerland
de - Germany
dk - Denmark
es - Spain
fi - Finland
fr - France
it - Italy
jp - Japan
nl - Netherlands
no - Norway
nz - New Zealand
ru - Russia
se - Sweden
uk - Ukraine
za - South Africa

Email address

Using an IP address or DNS address on the Internet, you can access any desired computer. If you would like to send a message by e-mail, then specifying only these addresses will not be enough, since the message must go not only to the right computer, but also to a specific user of the system.

A special protocol called SMPT (Simple Mail Transport Protocol) is used for delivering and receiving email messages. The computer through which email messages are transmitted to the Internet is called an SMPT server. By email, messages are delivered to the computer specified in the address, which is responsible for further delivery. Therefore, data such as the user name and the name of the corresponding SMPT server are separated by the “@” sign. This sign is called "at commercial" (in jargon - dog, dog). Thus, you address your message to a specific user of a specific computer. For example:
ivanov@site Here ivanov is the user to whom the message is intended, and the site is SMPT - the server on which his email mailbox is located. The mailbox stores messages received at a specific address.

URL (Uniform Resource Locator, unified resource locator) is the address of some information on the Internet. It has the following format:
resource type://node address/other information
The following types of resources are considered the most common:

Ftp:// ftp - server
gopher:// gopher menu
http:// WWW address
mailto:// email address
news:// UseNet newsgroup
telnet:// computer where you can log in using telnet

The resource portion of a URL always ends with a colon and two or three slashes. What follows is the specific address of the node you want to visit. It can be followed by a slash as a delimiter. In principle, this is quite enough. But if you want to view a specific document on a given node and know its exact location, you can include its address in the URL. Below are several URLs and their meanings:

Http://www..php main page of the information and educational portal [email protected]

ftp://ftp.microsoft.com/dirmap.txt file named dirmap.txt on Microsoft's ftp server

So, the following types of addresses are possible on the Internet.