Network services and network services. Understanding Network Communications

Internet– a single global network that connects a huge number of networks around the world (from the English InterNet - “internetwork”, “network of networks”). The Internet arose in the 60s in the United States as a result of experiments to create a viable network that could not be disabled by destroying one or more command posts with central computers.

The Internet is a decentralized network that does not have an owner or governing body (although each network included in it has an owner and a system administrator), functioning and developing through voluntary (including commercial) cooperation of various organizations and users on the basis of common agreements and standards (protocols). ). Registered and numbered Internet standards, protocols, specifications form RFC electronic document system(Request For Comments - request for clarification).

Organizations providing connection and provision of Internet services – providers(English Internet Service Providers, Internet service providers) are connected high-speed trunk channels(cable, fiber optic, satellite, radio relay). A separate computer or local network can connect to the provider via leased line(permanent connection) or by dial-up line(temporary connection via modem and regular telephone network). The first method is more expensive, but provides higher data transfer speeds.

The modem signal can be transmitted:

· via regular telephone channel – dial-up line;

· By dedicated telephone line;

· on the base ADSL technologies(English: Asymmetric Digital Subscriber Line - asymmetric digital subscriber line) over a regular telephone channel, without occupying it and allowing independent and simultaneous telephone conversations.

The data transfer speed through a dial-up telephone line is about 30 Kbit/s for analog telephone lines and 60–120 Kbit/s for digital ones. For dedicated telephone lines, transmission speeds are up to 2 Mbit/s, for fiber-optic and satellite communication lines – hundreds of Mbit/s.

Permanent connections, depending on the network equipment used and the type of cable channel, provide data transfer rates of up to 20-40 Mbit/s and higher.

The Internet works based on basic TCP/IP protocol, introduced in 1983. In fact, TCP/IP is a set of protocols and consists of several main layers. So, transport protocol TCP(Transmission Control Protocol) ensures that data is split into small packets ( segments) before shipping and assembly after delivery, and IP routing protocol(Internet Protocol) is responsible for selecting routes over various nodes and networks between the sender and recipient (possibly different for different packets from the same message). Data packets prepared using this protocol are called IP datagrams(or IP packets). They include segments prepared using the TCP protocol, to which the sender and recipient addresses are added.

These protocols also perform other tasks, for example, the TCP protocol includes functions not only of the transport layer, but also of the session layer, but do not fit completely into the layer separation of the OSI model, since they were developed before its advent.

Each of information services ( information services) Internet solves its problems using its application protocols, based on basic TCP/IP protocols. The most famous of them:

· "World Wide Web" www(from the English World Wide Web) makes it possible to move documents, books, news, photographs, drawings, training courses, reference materials, etc. in the information space; Currently, the WWW, in fact, claims to be the main carrier of the “collective memory” of humanity. The www service uses http protocol and is discussed in more detail below.

· Email or Email(from the English Electronic mail) allows you to exchange electronic letters over the network, which can be accompanied by additional files. Using the E-mail service, you can also send messages to cellular telephone, communicator, fax, pager. Used to send correspondence SMTP protocol(English: Simple Mail Transfer Protocol) to receive it from your mailbox on mail server – POP protocol(eng. Post Office Protocol - protocol post office). IMAP protocol(Internet Message Access Protocol) allows you to store mail in your mailbox on a mail server. To attach arbitrary files to a letter, use MIME standard(Multipurpose Internet Mail Extension - multi-purpose Internet mail extension). The rules for generating email addresses are discussed below. Used to work with email mailers Outlook Express (included with Microsoft Internet Explorer), Microsoft Outlook(included in Microsoft Office), Netscape Messenger (included in the Netscape Communicator browser), The Bat! and others clients E-mail.

· File transfer service between remote computers used to transfer large files (archives, books, etc.) via FTP protocol(English File Transfer Protocol - file transfer protocol). To work with FTP you need FTP client, which can be built into the Internet browser, file manager or supplied separate application. FTP clients differ in their ability to use multithreading(downloading parts of files in several parallel processes), support for “resuming” a file after a connection failure, restrictions on the maximum supported file size.

· Teleconferencing service (news, news groups) UseNet News (Newsgroups) provides viewing of materials on a selected topic sent to the teleconference server by the users themselves. Also used mailing lists, formed with the participation of the administrator ( moderator) conferences and sent to subscribed subscribers.

· Before the widespread spread of the Internet, the functions of teleconferencing were largely performed by electronic bulletin boards BBS(English Bulletin Board System - a system of electronic bulletins), the most famous system of which is the FidoNet network. Connection to the BBS is carried out over small computer networks with one server using modems through telephone lines.

· IRC online communication service(Internet Relay Chat - literally, Internet broadcast of chatter), which is often called chat conferences or simply chat, supports a collective conversation, the participants of which type their remarks on the keyboard and see what others have said on the monitor.

· Internet paging system ICQ(from the English “I seek you” - I am looking for you, “ICQ” in the jargon of domestic Internet users) makes it possible to exchange messages and files in real time. This system provides a search for the network address of a subscriber (permanent or temporary), if he is currently connected to the network, by his personal identification number UIN(English Universal Internet Number), received when registering on the central server of this service.

· Telnet service serves for remote control(By Telnet protocol) via the Internet by other computers and programs installed on them, for example, connected to equipment for conducting experiments or performing complex mathematical calculations.

· Important areas of using the Internet are Internet telephony (IP telephony)– transmission telephone conversations and faxes over the Internet in encoding corresponding to the IP protocol, broadcasting radio and television programs over the Internet, wireless connection to the Internet from mobile phones: directly via WAP protocol(Wireless Application Protocol), or via a computer via GPRS protocol(General Packet Radio Service).

· Encryption of information transmitted over the Internet is ensured SSL protocol (Secured Socket Layer).

Each computer connected to the Internet receives a unique (non-repeating) IP address(that is, an address corresponding to the IP protocol). With a permanent connection, this address is assigned to it; with a temporary connection, a temporary (dynamic) address is allocated for the session. In this case, a computer that is constantly connected to the network and through which temporary users connect is called host computer(from the English host - owner).

Physical IP address is a 32-bit (4-byte) binary number, which is usually written by converting each byte into a decimal number and separating them with dots. This number encodes the network through which the computer accesses the Internet and the number of the computer on the network. Depending on the permissible number of computers, networks are divided into three classes (Table 3).

Table 3. Network classes A, B, C

For example, the address 197.98.140.101 corresponds to host number 0.0.0.101 on the class C network 197.98.140.0.

To separate the network address from the host address, use Subnet mask, which is also a 32-bit number. By default, class A networks correspond to a mask of 255.0.0.0, class B – 255.255.0.0, and class C networks – 255.255.255.0, that is, in the binary representation of the mask, positions corresponding to the network address are closed with ones. The subnet mask can be used for other purposes, for example, to logically divide local networks into smaller subnets.

Several IP addresses are reserved for special purposes, e.g. address 127.0.0.1 provides the user with access to himself (used for testing programs and debugging Web applications on the local server). A network number with a computer number of 0 denotes this entire network, and with the maximum possible number (255 for network C) it is used for broadcast message, sent to all computers on the network.

It is more convenient for users to work not with physical addresses, but with domain names networks and computers on the Internet. This name consists of symbols separated by dots domains(from Latin dominium – possession.) – network fragments. From right to left, the most extensive is indicated, senior domain (first or top level), then lower domains nested within one another, and so on until the leftmost domain corresponding to the end node of the network. At the beginning of the domain name, before the server name, the Internet service in which this network node operates can be indicated (for example, www. - "World Wide Web" or ftp. - file transfer service). Domains of the third and lower levels are often called subdomains or subdomains.

Domains top level most often denoted by two (country) or three (type of organization) letters. Some of them are given in table. 4.

For example, microsoft.com – domain address Microsoft in the domain of commercial servers, and the domain cit.sibstrin.ru can mean a subdomain address local network center information technologies cit, which is a subdomain of the NGASU network (Sibstrin) in the domain of Russian-language servers ru.

Unique correspondence between physical and domain names is ensured by a special domain name system Internet - DNS (English Domain Name System), consisting of computers called DNS servers(each domain has a DNS server serving it). The user deals with domain names, and data transfer between computers is carried out using physical addresses that are automatically determined by contacting the appropriate DNS servers.

Table 4. Some top-level domains

At the top of the DNS server hierarchy are root zone servers with names a.root_servers.net, b.root_servers.net, etc., duplicating each other’s information. The local server, having received a connection request with a certain address from the client machine, passes it to the local DNS server, which will extract the domain name from the request and either find the corresponding IP in its database or contact one of the root zone servers. The latter will return a pointer to the DNS server of the domain known to it, which includes the requested address, and will completely eliminate itself from the process. Such nested queries can be repeated, with each time the local DNS server accesses the name server more and more low level. Only after this multi-step process is completed, the DNS server will return the translated address to the computer that made the request, and the user will finally be able to see on his monitor what kind of information is located at the address he entered.

Domain names and physical IP addresses are distributed by the International Coordination Center for Domain Names and IP Addresses (ICANN), which includes 5 representatives from each continent (Internet address www.icann.org).

To access a file (program, document) on the Internet, you need to specify a URL (Uniform Resource Locator), consisting of:

· the name of the protocol used to access the file and separated from the subsequent part by a colon and two forward slashes;

· computer domain name, separated from subsequent contents by a slash;

· the full name of the file on the computer (without specifying the logical drive), including (possibly) the access path (list of subdirectories), the actual file name and extension.

The URL can only use Latin letters (lowercase and uppercase letters are considered different) without spaces. The path and file name may be missing, which corresponds to access to the computer (server) itself.

For example, a URL like http://www.students.informatika.ru/library/txt/klassika.htm means that the klassika file with the htm extension is located in the txt subdirectory of the library directory on the students server of the informatika.ru domain. This server belongs to the www service, and the http protocol is used to access the file.

The address ftp://ftp.netscape.com/books/history.doc is used when retrieving the history.doc file located on the netscape server of the commercial Internet domain using the ftp file transfer protocol (ftp service).

Quite often you will come across URLs that do not contain the name of the html file, however, when you enter such a URL, you still get to a specific Web page. This means that the document has a default name, which can be assigned during server administration. Most often this name is index.html, so the URL http://www.host.ru can mean exactly the same thing as http://www.host.ru/index.html. The default http:// protocol prefix is ​​also usually omitted when writing the full URL.

To work with e-mail, you must register your account on one of the Internet mail servers. Mailbox, to which is assigned email address. This address consists of the server’s domain name and the login(the name of the mailbox, it is selected by the user during registration). These two parts of the address are separated by the symbol @ (read “et”, in Russia they often use the slang expression “dog”).

For example, [email protected]– mailbox of the subscriber who selected the director name on the contora.ru server.

As stated above, the leading and most widely used Internet service today is the World Wide Web (www), which covers a large volume of information resources. In this system it is easy to find news, reference and regulatory materials, books, articles, abstracts, software, opinions and expert advice on almost any topic. www also contains a huge amount multimedia content, such as graphics and animation, video and audio recordings, online games, etc.

The www service is based on the presentation of documents in the form hypertext– a text that allows not only sequential reading. The point is that hypertext elements, such as phrases, individual words, drawings may refer to other fragments of the same text or other documents, located, perhaps, on another computer on another server. The physical location of the server addressed by the link does not matter. Links ( hyperlinks, hyperlinks) are usually marked with a special color and font, and are navigated automatically after clicking on the label. Thus, various information turns out to be interconnected by an intertwining web of links, and the collective knowledge of humanity introduced into the system is to a certain extent likened to individual memory, woven into one whole by associations and semantic connections.

The hypertext-based www concept was developed in 1989 by the English scientist Timothy Berners-Lee for the European Particle Physics Laboratory, based in Switzerland and uniting physicists from around the world. The concept of hypertext itself was proposed by the American scientist Theodore Holm Nelson in 1965.

A document presented on the WWW is called Web page, and the computer on which such documents are located is Web server. Web pages are created using hypertext markup language HTML(English HyperText Markup Language) or more powerful XML language (English e X tensible M arkup L anguage is an extended markup language), there are other markup formats.

Typically, the markup format allows you to define hyperlinks and text organization by including control characters - tags(from the English tag - tag, label). The formatting of a Web page on a monitor is determined both by tags that control markup and by specific computer settings. You can place pictures on Web pages in one of the three main Web graphic formats: gif, jpg(jpeg), png, multimedia objects (flash animation, sound and video files), forms for dialogue with the user, controls ( ActiveX), launching programs. Such programs are most often written in a programming language Java (Java), designed to support Web pages. Translators from this language are interpreters, which allows you to write universal programs that run on different computers and different operating systems.

Used to access web pages Hypertext Transfer Protocol HTTP(Hyper Text Transfer Protocol).

Viewing Web pages and moving between them in the network information space using hyperlinks ( web navigation) provide special programs Web browsers ("navigators", the most common name is browsers, from English browsе – look, scroll through). Browsers are the main programs - clients service www. The most commonly used browsers nowadays are Mozilla Firefox,Opera, Google Chrome(Google company), Safari, Internet Explorer (Microsoft company). In the recent past, there were only two popular browsers - Internet Explorer and Netscape Navigator(Netscape company).

Browsers have continually evolved since the dawn of the WWW, becoming an increasingly important program on the typical personal computer. A modern browser is a complex application both for processing and displaying various components of a web page, and for providing an interface between the website and its visitors. Almost all popular browsers are distributed free of charge or bundled with other applications, for example, Internet browser Explorer is part of the operating room Windows systems, Mozilla browsers Firefox and Opera latest versions are free programs, Safari browser distributed as part of the operating room Mac systems OS.

Control of any modern browser is quite standardized. At a minimum, the following tools are required to work comfortably in the browser:

· address bar(address bar, navigation bar, Toolbar) contains and allows you to enter the URL of the required page or the path to a locally located document, and also places standard page navigation buttons ("Forward", "Back", "Refresh", "Stop", "Home"). In some browsers, standard buttons are placed on a separate toolbar;

· status bar (status line) - the bottom information field of the browser window containing important Additional information. Thus, while a web page is loading, information about its progress is displayed in the status bar, and when you hover the mouse cursor over a link, the URL corresponding to the link is displayed in the status bar;

· tab bar(sometimes the bookmarks bar, Tab bar) - allows you to open additional web pages in the current window and switch between them. The concept of tabs allows you to more conveniently manage sets of simultaneously open web pages without giving up the ability to open a link in a new browser window.

These toolbars are usually enabled out of the box and can be controlled from the browser's View menu.

As a rule, apart from personal preferences, most often Internet users work with sites search engines. Their use is very simple - search engines Web servers produce a selection of all www documents known to them, containing keywords from the user’s query, and the query is made in natural language. The most famous and effective in Runet(Russian-language segment of the Internet) search engines - Google, Yandex and Mail.Ru.

The speed of searching for information in such systems is ensured by work invisible to the user special programs(“search robots”) that continuously scan various websites and update lists of terms found on them ( search engine indexes). Thus, in reality the search takes place not on “all Internet servers,” which would be technically impossible, but on the search engine database, and the absence of suitable information found upon request does not mean that it is not on the Internet - you can try using another search tool or resource directory. Search server databases are not only replenished automatically. On any major search engine It is possible to index your site and add it to the database. The advantage of the search server is the ease of working with it, the disadvantage is the low degree of selection of documents upon request.

Both search servers and individual Web site developers also form rubricators or catalogs– hierarchical structures of topics and concepts, moving through which the user can find necessary documents or websites. The catalog is usually replenished by the users themselves after checking the data they entered by the server administration. A resource catalog is always better organized and structured, but it takes time to find the right category, which, moreover, is not always easy to define. In addition, the volume of the catalog is always significantly less than the number of sites indexed by the search engine.

Web sites can also be classified in terms of the technology used to develop them. The phrase “HTML language,” historically established in the Russian language, does not reflect the fact that HTML and XML are not programming languages. However, more often than not, a modern Web page is dynamic, that is, is the result of a server program that generates a page in response to a user request for one or another URLs(Unlike static pages in HTML markup, stored on the server as a file with the extension .htm or .html). The main server programming languages ​​are PHP, Perl, Python and a number of others. There are also client Web programming languages ​​such as Javascript and VB Script. A program in such a language, included in the text of a Web page, is executed not on the server, but on the client computer, using an interpreter included in the user’s browser or installed separately.

Starting a conversation about building corporate networks, first you need to decide on basic concepts, which will be used further.

The first of these is the concept of “network” (net, or network). This word in Russian has many different meanings and is used to determine various interactions. The network can be telephone or fishing, it can be computer or radio broadcast. We will talk about building corporate networks that connect an organization's computers, which are most often called computer, or computing, networks. What is a computer network? Probably the simplest and most general definition will be the following: a network appears when two or more computers (and in fact, the users of these computers) have something to share. Sharing refers to the sharing of resources. The process of separation (sharing) itself

network resources called networking. Resource sharing can be done

different ways

, depending on the available computer facilities. The first method of interaction involves completely centralized processing of information and its storage, providing users with work from terminals. This interaction model is often called “terminal-host”. The user interacts with the resources of the central computer, using its processor, RAM and disk memory, as well as peripherals. In this case, very often the user does not work alone, but together with other users, that is, the resources of the central computer are used in a shared mode.

Central computer must run under an operating system that supports such interaction, which is called centralized computing. The further development of the computer industry went in different ways, the computing power of computers designed to work on “terminal-host” interaction increased, personal computers appeared and began to develop rapidly.

Computers are connected into a computer network. Tasks are distributed among computers on the network, which allows you to expand functionality each of them by sharing access to other computers.

Currently, the task of combining distributed computer resources to perform (solve) a common problem is relevant and rapidly developing. This interaction model is called collaborative computing. In this case, the task is distributed among computers, computers exchange common data with each other, the total computing power and available resources (RAM and disk memory) increase, and the fault tolerance of the entire system as a whole increases from the point of view of solving the problem. As a rule, distributed execution of a task is controlled by a special control system

, which, if one of the computers fails, will shift the execution of its part of the work to the remaining computers. Relatively new model network interactions is the organization of interactions between network users and network services. From the user's point of view, his relationship with many computers falls under the definition of "client-network". For a network user, in general, it is not important where exactly on the network the resources allocated to him are located; he must only be able to access them using the system of calls adopted in the network. With this approach, the work of all network users is significantly simplified, and the network resources and services themselves should be available to the user at any time. Increasing the level of availability of network services requires appropriate

technical solutions , for example, increasing fault tolerance or duplicating services. There are many different components present. The most visible to network users are two. This is a network server and a client. The server (server - literally translated from English means “the one who serves”) of the network is designed to serve requests coming from the client of the network. In other words, the client always requests service, and the server always serves the client. In some cases, a client can also act as a server, processing requests from other clients and requesting service from other servers. Based on the way servers and clients interact, two types of networks are defined: “client-server” and “peer-to-peer”. Since the network client is a user working on a computer, he himself user's computer

, connected to the network, is defined by the term “workstation” (workstation).

This term is used on a par with the term “computer”. Often the client/server and peer-to-peer models can exist simultaneously on the same network. Networks built on the peer-to-peer principle are also called peer-to-peer networks, in which all computers have the same status - rank. The following are considered classic services in networks: file, print, messages, applications and

databases . The most important of them were and remain the file service and the print service., File service provides organization tasks remote access sharing, quick transfer and replication, Reserve copy files. This service provides centralized

file storages

, efficient use of their disk systems.

An application service allows users to share not only data (as in a file service), but also the processing power of the server to complete tasks. In this case, the user's task is executed on the server processor. The application server is specialized, it is optimized to perform specific tasks and must support the possibility of further increasing its computing power.

The database service is designed to organize centralized storage, search and ensure data protection. This service is implemented by database servers, software and hardware systems optimized for performing the listed tasks, reducing user access time to information, and managing the territorial location of information on the network.

General information about network devices...

Other network components are the means for organizing a data transmission channel between clients and network servers. IN general case a data transmission channel is built using the following components: a data transmission medium - wired or wireless (wireless) - and interface cards (network interface card, NIC), which ensure the interaction of the computer with the data transmission medium. However, these are not the only means used to connect computers and form the computer network. Network hardware and hardware-software help connect computers into a network and ensure their interaction. These funds can be divided into groups according to their main functional purpose: connectors, repeaters, adapters, modems, bridges, hubs, switches, routers.

The most commonly used wired data transmission media in networking are created using cable connections that use either a metal conductor for electrical signals or a fiber-optic conductor for light signals.

Wireless information transmission media provide for the organization of interaction between computers through the transmission of light (infrared) and radio frequency signals.

With the help of data transmission media and some hardware and software for intercomputer interaction, the physical topology of the network is formed, physical connections of all computers and other network tools are made.

ComputerPress 3"1999

Internet protocols

On the Internet, as on any other network, there are seven levels of interaction between application processes, and each level corresponds to a set of protocols (i.e., rules of interaction).

Protocols physical level determine the type and characteristics of communication lines between computers. For each type of communication line, a channel (logical) level protocol has been developed to control the transmission of information over the channel. For example, link-layer protocols for telephone lines include SLIP (Serial Line Internet Protocol) and PPP (Point to Point Protocol). For communication via a local network cable, these are package drivers for LKS boards.

Protocols network layer provide routing of packets in the network, i.e. are responsible for transferring data between computers on different networks. These include IP and ARP (Address Resolution Protocol).

At the transport layer, data transfer is controlled by the TCP and UDP (User Datagram Protocol) protocols. Session layer protocols are responsible for establishing, maintaining, and destroying appropriate channels. On the Internet, this is done by the TCP, UDP, and UUCP protocols.

At the representative level, protocols are concerned with serving application programs. Representative-level programs include, for example, programs that run on a Unix server to provide various services to users. These are telnet server programs, FTP server, Gopher server, NNTP (Net News Transfer Protocol), SMPT (Simple Mail Transfer Protocol), etc. Application level protocols include programs for providing network services.

All Internet services can be divided into three groups - interactive, direct access and delayed reading.

The group of interactive services includes those that require an immediate response from the recipient of information, i.e. the information received is essentially a request.

Direct contact services are characterized by the fact that information upon request is returned immediately.

The most common are delayed reading services, such as email. For them, the main feature is the fact that the request and receipt of information can be quite separated in time (this is determined by the relevance of the information at the time of its receipt). Lazy reading services are the most universal and least demanding of computer resources and communication lines.

There is another approach to dividing the services provided by the Internet. They are divided into two categories: services for the exchange of information between network subscribers and services related to the use of network databases.

Let's look at the most common network services.

Email is a typical off-line service. An email consists of a header containing the address of the sender and recipient, and the actual body of the letter. Each user in the electronic signature system is allocated a mailbox, implemented as a file on disk, where the forwarded message from another user is placed. Emails are retrieved from the mailbox using the appropriate commands.

The Internet standard e-mail system is universal: networks built on completely different principles and protocols can exchange e-mails with the Internet, thereby gaining access to its other resources. Almost all Internet services, usually used as direct access services (on-line), have an interface to e-mail. Therefore, if a user does not have on-line access to the Internet, he can obtain most of the information stored on this network through cheap email.

The Internet allows you to send both text and binary files. There is a limitation on the size of an email message on the network: it should not exceed 64 kilobytes.

Network news (teleconferences) is the second most common Internet service. The mechanism for distributing network news is quite simple: each network node that receives a new message transmits it to those nodes with which it exchanges news. Consequently, the message sent by the user is distributed, duplicated many times, throughout the network, reaching in a short time all participants in USENET newsgroups around the world.

News is divided into hierarchically organized thematic groups. The name of each group consists of the names of the sublevels of the hierarchy, separated by periods, with the more general level written first. There are global hierarchies and hierarchies that are local to an organization, country, or network. The set of groups received by a local USENET server is determined by the administrator of this server and the presence of these groups on other servers with which news is exchanged local server. Typically the server receives: all global hierarchies; groups local to the country in which the server is located; groups local to the organization where the server operates. Different norms and rules for working with them apply to different hierarchies. This concerns primarily the language of messages. It is better to write messages to groups of the Russian relcom hierarchy in Russian, while to groups of the local comp hierarchy you should write only in English.

Any computer fully connected to the Internet has access to USENET news, but USENET news is distributed over other networks.

The convenience of working with news depends significantly on the method of receiving it. On the Internet, a subscriber's client program can directly receive news from a USENET server, and then there is no delay between viewing the list of messages contained in a group and reading those messages. If the news is used via e-mail, then the subscriber first receives a list of articles, and only then receives the articles ordered from this list by e-mail. This is a very inconvenient and outdated way of working with USENET news, which, however, is the most common in Russia.

FTP (File Transfer Protocol) is a file transfer protocol. This is not just a protocol, but a service - access to files in file archives. FTP is standard program, working according to the TCP protocol. It ensures file transfer between computers interacting on TCP/IP networks: on one of them the server program runs, and on the other the user runs a client program that connects to the server and transfers or receives files via the FTP protocol.

FTP is a direct access service that requires connecting your computer to the Internet. However, access is also possible via e-mail, for which there are servers that, upon request, can send the requested files by e-mail. In this case, the request can wait quite a long time for its turn. There is another inconvenience: when large files are sent, they are divided by the server into parts of limited size, sent as separate letters; If at least one letter is lost, the remaining received letters belonging to the requested file will be unnecessary.

Systems for automated information retrieval on the Internet. Companies around the world widely use the Internet, the global information superhighway, to search for and obtain information of almost any kind. There are thousands of databases and dozens of navigation systems. To facilitate and speed up the search for the necessary information, auxiliary programs are used that are integrated into the Internet structure and form the core automated systems searching and obtaining information.

The Internet works with three main information retrieval systems - Gopher, Wais and WWW.

Gopher hypertext system. This is a fairly well-known and widespread tool for searching information on the Internet, allowing you to find information on keywords and phrases. When working with Gopher, the user is prompted to navigate through a series of submenus from which files are available various types. Gopher, being a distributed structured information export system, is a direct access service and requires that both the server and client be fully connected to the Internet.

The Gopher system allows you to obtain information without specifying the names and addresses of the authors. The user simply tells the system what exactly he needs, and the system finds the necessary data.

WAIS system. This is a wide-profile information system, which is a set of programs designed to index large volumes of unstructured (usually just text) information, search through such materials and extract the requested data from them. These functions are performed using indexing programs, local search programs based on the obtained indexes, as well as server and client programs that interact with each other using a special Z39.50 protocol.

The task of searching for data in large volumes unstructured information is very non-trivial, until there is no generally accepted solution to it. The WAIS system implements an acceptable solution to this problem, so it has gained sufficient popularity as one of Internet services. However, in Lately this system is almost never used on its own, and in many cases it is used as an auxiliary tool, for example, for indexing documents stored on a WWW server.

WWW (World Wide Web) system information network). WWW is the most popular and convenient means of working with information. More than half of the data flow on the Internet comes from the WWW. The number of WWW servers today exceeds 30 thousand. WWW is a hypertext, hypermedia, distributed, integrated, global decentralized information system that implements the most advanced and widespread technology. This is a direct access service that requires a full Internet connection. WWW works on the client-server principle. There are many servers that, at the request of the client, present him with a hypermedia document consisting of parts with a diverse representation of information (text, sound, graphics, three-dimensional objects, etc.). WWW software tools are universal for various Internet services, and the system itself plays an integrating role. The connection between the client and the WWW server is one-time: after receiving a request from the client and issuing a document to him, the server interrupts the connection.

WWW is a union in one information system capabilities of the above information tools with the addition of transmission (in addition to texts and programs) graphic images, sounds, video. All these information objects are connected by the hypertext structure, i.e. text containing connections with other texts, graphic, video or audio information. The WWW system is distinguished by the following features: the use of hypertext and the ability of users to interact with other Internet applications.

Hypertext can be thought of as a system of cross-referenced documents. Communication between hypertext documents is carried out using keywords, and the documents to which links are made can be located on remote computers. Consequently, using the links you can move significantly away from the original source of information, but returning to it does not cause difficulties.

Hypermedia documents (i.e. hypertext documents that include not only texts, but also graphics, sound and video) are stored on WWW servers on the Internet. There are many different client programs available for working with hypermedia documents, called WWW viewers or browsers. Using a known address, browsers allow you to call up the necessary documents, accumulate them, sort them, merge them, edit them, and print them. The most widely used viewing programs at present are Netscape Navigator and Microsoft Internet Explorer. Viewer programs have a lot in common, therefore, having mastered the principles and tools of one of them, you can easily switch to working with another. Majority modern programs Viewers provide access not only to Web server pages (or Web pages), but also include the ability to process e-mail, Usenet newsgroups, allow you to work with the FTP service, Gopher, etc. Web page editors are built into viewer programs.

The preparation of hypermedia documents is carried out in HTML (Hyper Text Markup Language - a language for describing hypertext documents). HTML is the World Wide Web language used by every Web site to display information. It was developed in the early 90s by an initiative group at the European Particle Physics Laboratory in Geneva. An HTML Web page (also called a Web document) is a simple text file (ASCII file) that you can create and read. It contains a set of HTML commands that tell the browser the display order Web pages. Therefore, after connecting to a Web site using a browser, the requested document in HTML format is sent through the communication network from the Web server to the browser. Any computer (regardless of whether it is a regular PC running Windows, workstation Unix-based or Macintosh computer) can accept and display HTML pages. This explains the effectiveness and popularity HTML language. There are a large number of programs that convert various documents in HTML format.

The Internet implements two sides of information search, different in methods, but common in goals: directories and search servers. Conventionally, we can say that catalogs are a means of focused information search, and search servers are a means of scattered search. Using these tools allows you to quickly and effectively find necessary information on the global network.

Search servers (search engine) are special software that, by automatically scanning all Internet resources, can find the requested resources and index their content. The user submits to the search server a phrase or set of keywords that describe a topic of interest to him. By making such a request, the server provides the user with a list of matching resources. There are many search servers on the Internet, covering almost all available resources. In this case, different servers cover different, partially overlapping, areas of information on the network. They use different methods for indexing documents and ways of assessing the significance of words in them. There are specialized search servers for specific types of network resources and universal ones that cover all types of services.

Internet directories are means of storing thematically systematized collections of links to various network resources, primarily WWW documents. Links to such directories are entered by administrators who try to make their collections as complete as possible, including all available resources on each topic. As a result, the user should find the question he is interested in in the directory, and he does not need to collect all the links on this topic himself, since the work of searching and systematizing links has already been done. Directories usually have a tree structure and are similar to a very large list of bookmarks. Catalogs provide a variety of additional service: search by keywords in your database, providing lists of the latest arrivals, automatic notification by e-mail about new arrivals, etc. There is a catalog of Russian-language resources of the Internt network.

A subscriber who has learned to use the directory that is most suitable for him and several search servers receives an effective means of quickly finding information on the network.

Organizations that determine the direction of development of the Internet:

1) Internet Community,

2) Internet Engineering Problem Group,

3) WWW Consortium,

4) Registration group.

Basic Internet services:

1. Email mail.

2. WWW is a hypertext system for transmitting web pages.

page– the smallest unit of WWW information transmission.

website– a collection of web pages belonging to one company or individual.

portal– a group of sites with a specific set of services for the user.

3. FTP is a file transfer protocol.

4. Telnet – designed to control remote computers.

5. Gopher is an analogue of www, allowing you to search for information using a menu system that looks like a data tree.

6. IRC is a service that allows network users to exchange text messages in real time.

Internet addressing system:

IP address is a unique numeric address that uniquely identifies a node, group of nodes, or subnetwork on the Internet.

The uniqueness of network addresses is guaranteed by special. organization called network information center.

Domain Name System (DNS) – a service that allows you to convert IP addressing to domain and vice versa (i.e. 657621418óyandex.ru)

domain3. domain2. domain1.

First level domains:

Com-commercial organizations

Edu-educational organizations

Gov-government

Basic Internet services

The constant development of information technologies leads to the emergence of a variety of information resources that differ from each other in the forms of presentation and methods of processing their constituent information objects. Therefore, currently there are quite a large number of services on the Internet that provide work with the entire range of resources. The most famous among them are:

electronic mail (E-mail), which allows one person to exchange messages with one or more subscribers;

teleconferences, or news groups (Usenet), providing the possibility of collective messaging;

FTP service – a file archive system that provides storage and transfer of files of various types;

Telnet service, designed to control remote computers in terminal mode;

World Wide Web (WWW, W3) is a hypertext (hypermedia) system designed to integrate various network resources into a single information space;

DNS service, or domain name system, which provides the ability to use mnemonic names instead of numeric addresses to address network nodes;

IRC service designed to support real-time text communication (chat);

The services listed above are standard. This means that the principles of constructing client and server software, as well as interaction protocols are formulated in the form of international standards. Therefore, software developers practical implementation are required to comply with general technical requirements.

Along with standard services, there are also non-standard ones, which are the original development of a particular company. As an example, we can cite various systems such as Instant Messenger (original Internet pagers - ICQ, AOl, Demos on-line, etc.), Internet telephony systems, radio and video broadcasts, etc. An important feature of such systems is the lack of international standards, which can lead to technical conflicts with other similar services.

Standard transport protocol ports for services

For standard services, the interface for interaction with transport layer protocols is also standardized. In particular, each software server is reserved standard rooms TCP and UDP ports, which remain unchanged regardless of the features of a particular proprietary implementation of both service components and transport protocols. Client software port numbers are not so strictly regulated. This is due to the following factors:

firstly, several copies of the client program can function on the user node, and each of them must be uniquely identified by the transport protocol, i.e. each copy must have its own unique port number;

secondly, it is important for the client to regulate the server ports in order to know where to send the request, and the server will be able to respond to the client, having learned the address from the received request.

The table below lists the standard port numbers for the main services.

The set of server and client parts of the OS that provide access to a specific type of computer resource via a network is called network service. In the example above, the client and server parts of the OS, which together provide access through the network to file system computer, form a file service.

It is said that a network service provides network users with a certain set of services. These services are also called network service(from the English term “service”). Although these terms are sometimes used interchangeably, it should be borne in mind that in some cases the difference in the meaning of these terms is fundamental. Further in the text, by “service” we will understand a network component that implements a certain set of services, and by “service” we will mean a description of the set of services that is provided by this service. Thus, a service is an interface between a service consumer and a service provider (service).

Each service is associated with a specific type of network resource and/or a specific method of accessing those resources. For example, a print service provides network users with access to shared printers on the network and provides a print service, while a mail service provides access to information resource networks - emails. For example, the remote access service differs in the method of accessing resources - it provides users of a computer network with access to all its resources through dial-up telephone channels. To gain remote access to a specific resource, such as a printer, the Remote Access service communicates with the Print service. The most important ones for network OS users are the file service and the print service.

Among the network services, we can highlight those that are aimed not at the average user, but at the administrator. Such services are used to organize the operation of the network. For example, the Bindery service of the Novell NetWare 3.x operating system allows the administrator to maintain a database of network users on the computer running that OS. A more progressive approach is to create a centralized help desk or, in other words, a directory service, which is designed to maintain a database not only of all network users, but also of all its software and hardware components. Novell's NDS is often cited as an example of directory services. Other examples of network services that provide services to the administrator are a network monitoring service, which allows you to capture and analyze network traffic, a security service, the functions of which may include, in particular, performing a login procedure with password verification, and a backup and archiving service.

The rich range of services an operating system offers to end users, applications, and network administrators determines its position in the overall range of network operating systems.

Network services are, by nature, client-server systems. Since when implementing any network service, a request source (client) and a request executor (server) naturally arise, any network service contains two asymmetric parts - client and server. A network service can be represented in operating system either both (client and server) parts, or only one of them.

It is usually said that the server provides its resources to the client, and the client uses them. It should be noted that when a network service provides a certain service, the resources of not only the server, but also the client are used. The client can spend a significant part of its resources ( disk space, CPU time, etc.) to maintain the operation of the network service. The fundamental difference between a client and a server is that the client is always the initiator of the work performed by the network service, and the server is always in a passive mode of waiting for requests. For example, a mail server delivers mail to the user's computer only when a request is received from a mail client.

Usually the interaction between the client and server parts is standardized, so that one type of server can be designed to work with clients different types, implemented different ways and even from different manufacturers. The only condition for this is that clients and server must support a common standard protocol for interaction.