What is a global computer network definition. Global computer network Internet. Domain name system
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 in postal correspondence, Every packet sent over the network must have a destination address and a source 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. On 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: an enterprise identifier and a domain identifier (domain top level), which are 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 communication node (gateway) must know the addresses of several DNS servers in order to convert user-supplied 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.
One of the means of interaction between users in networks is electronic mail (e-mail). C Email The creation of the Internet began and it remains the most popular 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. Further we'll 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 sender user, the transmission of the message, the direction of the message to Mailbox addressee and storing it in this box 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, part of the Outlook office application;
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;
usage 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, and storing related messages. 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:
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 not required.
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
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
Development trend modern technologies communications 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're talking about not about use mobile phone as a switching device, in other words, a modem, but about its use 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 required 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 conversations “in live” 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 linked by the structure of hypertext.
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 presented 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, to select the desired addresses, contacts search servers accessible through Web interface. 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
Today, many users are increasingly faced with the concept of a global computer network. True, not everyone is fully aware of what it is in the broadest sense, and what the capabilities of the global network are, limited to just the Internet. Let's try to figure it out this issue in a little more detail, and also consider some of the main characteristics that are inherent in such computer structures.
What is a global network: a general concept
Let's start by understanding the very definition of networks of this type. Based on what is proposed in the description by the most famous and respected information sources on the World Wide Web, global networks are understood as organizational structures that connect individual computers or terminals located on a local network with each other, regardless of their physical location. So what is it?
Indeed, this is a certain structure that is capable of ensuring interaction between user terminals or even mobile devices, no matter where in the world they are located. What is most interesting is that such structures refer to virtual concepts, since wired connections between all devices around the world cannot be simply physically established.
Local and global networks: what's the difference?
Some users mistakenly believe that there is no difference between these two concepts. Here it is worth looking at the most important difference between both types of networks.
The local network itself is designed to connect only a strictly defined number of computer devices and cannot interact between them if their number exceeds. In addition, such networks provide only general access to some programs or documents, and communication is carried out through a central server or several servers.
The organization of global networks in this regard is fundamentally different. They can include individual computers or mobile devices, and entire local networks. In other words, there are no restrictions on the number of simultaneously connected devices (except perhaps by assigning an external identifier to each device, such as an IP address on the Internet or a mobile phone number). The IPv4 protocol will soon exhaust its capabilities due to the limited number of assigned addresses, but the sixth version, which is replacing the fourth, has such restrictions, if any, then they are very conditional.
Principles of organization
The development of global networks is believed to have begun from the moment when they tried to establish communication between computer devices via ARPANET. This network is fundamentally the progenitor modern Internet.
Only at the dawn of the implementation of such an idea, communication was carried out via cables, but over time, solutions for organizing computer interaction reached new level. If we talk in simple language, the structure is such that on one side there is a LAN router for output, and on the other there is a switch for communication with the required parts of the WAN.
Types of WANs
If we talk about what a global network is, we cannot help but touch upon the issue of modern types of such computer structures.
Basically, the classification distinguishes several main classes, among which any user knows the following:
- satellite networks;
- mobile networks;
- Internet and its varieties.
How it works?
As is already clear, access to the global network is provided through device identification, and communication is carried out through the use of special protocols.
The protocols themselves may vary for different networks and different operating systems, but in international standards you can usually find protocols like TCP/IP, ATM, MPLS, SONET/SDH, etc. Each such protocol is a set of specific rules by which access to the global network, information is transmitted and received, or user devices are identified, etc. Note that in this case we are not talking about initializing the user’s persona. All this applies exclusively to computers or mobile devices.
The most famous global networks
In general, today the most popular networks are considered to be the Internet and FidoNet. However, few people realize that networks mobile operators They are also unique global structures that use GSM technology standards for communication between devices.
What about 3G/4G? Here you need to clearly understand that these standards are used exclusively for accessing the Internet, and, more simply, for connecting one global network to another. And any global network is initially focused on high data transfer speeds, which distinguishes it from a local structure. But today, the networks of mobile operators can equally be classified as both a local and a global network, since they unite only strictly defined devices identified by numbers, and on the other hand, their number is growing day by day, which implies the assignment of such identifiers in almost unlimited quantities.
Some Basic Features and Challenges
But let's see what the global Internet is. It is the structure called the World Wide Web that has become the most popular, developed and extensive. If previously it was focused mainly on sending correspondence in the form of e-mail or visiting web pages, today its resources are such that users anywhere in the world can communicate with each other, say, through video chats in real time or in social networks, upload information of any type, store your own data in cloud services, etc.
One of the most interesting tools is simultaneous access to electronic documents, which involves opening and editing files by several users at once. It goes without saying that any change in the document is immediately displayed on the computers of all connected to the this moment users. What is a global network in this sense? This is a tool that provides software interaction at all levels and between any users.
But the emergence of the World Wide Web, in a sense, gave rise to many problems, since it is on the Internet today that such a huge number of viruses, malicious codes and programs are distributed that it is difficult to imagine. Even the most advanced antivirus software developers are unable to keep up with them.
Of course, these are not all the possibilities that can be cited as an example. Bitcoin mining, which has been gaining momentum recently, can also be classified as such a tool. Here the technology is such that via the Internet you can combine into one virtual network machines even without the consent of their owners and take advantage of a multiple increase in the performance of a single computer by using the computing capabilities of other terminals. Naturally, in a sense, such programs can be called viruses or actions that fall under the jurisdiction of illegal access to other people's information, however, precisely as tools of global networks, such opportunities cannot be discounted.
In addition, it is worth special mentioning network operating systems that do not require installation on HDD, or can be downloaded to a computer terminal from remote server, providing full-time job any device. It is believed that such technologies are the most relevant today, since the security system used for their structures and remote access is much higher than in stationary systems.
Brief conclusions
In general, I think it is already a little clear what a global network is and how it differs from a local network. Naturally, it is impossible in principle to consider absolutely all the tools provided. However, this, in fact, was not the question. At least from the above material you can understand what these structures are, why they are needed and what basic capabilities they have.
Initially, global networks solved the problem of accessing remote computers and terminals to powerful computers called host computers (the term server is often used). Such connections were made through switched or non-switched telephone network channels or through satellite dedicated data networks, for example, networks operating using the X.25 protocol.
To connect to such data networks, modems were used, running special telecommunication programs such as BITCOM, COMIT, PROCOM, MITEZ, etc. These programs, running under the MS-DOS operating system, ensured the establishment of a connection with a remote computer and the exchange of information with it.
With the end of the MS-DOS era, their place is taken by communications software built into operating systems. An example would be Windows95 tools or Remote Access Service (RAS) in WindowsNT.
Currently, single computers connected to global networks are used less and less often. These are mainly home PCs. For the most part, subscribers of computer networks are computers connected to local area networks (LAN), and therefore the problem of organizing the interaction of several remote local area networks is often solved. In this case, it is necessary to ensure that the remote computer can communicate with any computer on the remote local network, and, conversely, any computer on the LAN with the remote computer. The latter becomes very relevant when expanding the fleet of home and personal computers.
In Russia, the largest global networks are considered to be the Sprint network (modern name Global One), the Infotel network, the Rosnet and Rospak networks operating using the X.25 protocol, as well as the Relcom and Internet networks operating using the TCP/IP protocol.
Switching centers are used as network equipment, which for X.25 networks are often designed as specialized devices from manufacturers Siemens, Telenet, Alcatel, Ericsson, etc., and for networks with TCP/IP, routers from Cisco and Decnis are used. The network structure is shown in Figure 6.
Figure 6 - The principle of connecting computers in global networks.
3.2 Internet
The Internet is the oldest global network. The Internet provides various ways for remote computers to interact and share distributed services and information resources.
The Internet operates using the TCP/IP protocol. The main “product” you can find on the Internet is information. This information is collected into files that are stored on host computers and can be presented in a variety of formats. The data format depends on what network service you used and what capabilities the PC has for displaying information. Any computer that supports TCP/IP protocols can act as a host computer.
The key to obtaining information on the Internet is resource addresses. You will have to use mail addresses when forwarding emails to your colleagues and host names to connect to them and receive files of information.
One of the disadvantages of data transmission over the Internet is insufficient information security.
ServicesInternet.
Transfer files via FTP protocol. An information service based on file transfer using the FTP (File Transfer Protocol) protocol.
Searching for files using the Archie system. Archie is the first search system necessary to find the necessary information scattered on the Internet.
Email. ES is a type of network service. ES provides for the transfer of messages from one user with a specific computer address to another. It allows you to quickly contact each other.
Teleconferences. Internet newsgroups provide the opportunity to conduct discussions (via messages) on thousands of posted topics.
Network capabilitiesInternet.
The Internet is a global computer network containing a huge amount of information on any topic, available on a commercial basis to everyone, and providing a wide range of information services. Currently, the Internet is an association of more than 40,000 different local networks, for which it is called a network of networks. Each local network is called a node or site, and the legal entity that ensures the operation of the site is called a provider. The site consists of several computers - servers, each of which is designed to store information of a certain type and in a certain format. Each site and server within the site have unique names by which they are identified on the Internet.
To connect to the Internet, the user must enter into a service contract with one of the providers in his region.
Access to information resources.
There are several types of information resources on the Internet, differing in the nature of the information, the way it is organized, and the methods of working with it. Each type of information is stored on a server of the corresponding type, called by the type of information stored. Each information system has its own means of searching for the necessary information throughout the Internet using keywords. The following information systems operate on the Internet:
World Wide Web (WWW) – World Wide Web of Information. This system is currently the most popular and dynamically developing. Information on the WWW consists of pages (documents). Pages may contain graphics, be accompanied by animation of images and sound, played directly as information arrives on the user’s screen. Information on the WWW is organized in the form of hypertext. This means that there are special elements in a document - text or pictures, called hypertext links (or simply links), which, when clicked with the mouse, display another document to which the link points. Wherein new document may be stored on a completely different site, perhaps located on the other side of the globe.
Gopher system. This system is the predecessor of the WWW and is now losing its significance, although it is still supported on the Internet. Viewing information on the Gopher server is organized using a tree menu, similar to the menu in Windows applications or similar to a tree of directories (folders) file system. The top-level menu consists of a list of large topics, for example, economics, culture, medicine, etc. The menus of the next levels detail the selected menu item of the previous level. The final point of movement down the tree (a leaf of the tree) is a document, just as the final element in a directory tree is a file.
FTP (File Transfer Protocol) is a system used to transfer files. Working with the system is similar to working with the NC system. Files become available for work (reading, execution) only after copying to your own computer. Although file transfer can be done using the WWW, FTP systems continue to be very popular due to their speed and ease of use.
Internet addressing and protocols.
A computer connected to the Internet and using a special TCP/IP protocol to communicate with other computers on the network is called a host. To identify each host on the network, there are the following two addressing methods, which always work together.
The first addressing method, called an IP address, is similar to phone number. The host IP address is assigned by the ISP, consists of four groups of decimal digits (four bytes), separated by periods, ending with a period.
Similar to telephones, each computer on the Internet must have a unique IP address. Usually the user does not use his IP address. The disadvantage of an IP address is that it is impersonal, lacks a semantic characteristic of the host, and is therefore difficult to remember.
The second way to identify computers is called a domain name system called DNS (Domain Naming System).
DNS names are assigned by the provider and, for example, look like:
win.smtp.dol.ru.
The above Domain name consists of four, separated by dots, simple domains (or simply domains). The number of simple domains in a fully qualified domain name can be arbitrary. Each of the simple domains characterizes a certain set of computers. The domains in a name are nested, so that any domain (except the last one) is a subset of the domain following it to the right. So, in the given example of a DNS name, the domains have the following meaning:
ru– country domains, in this case means all domains in Russia;
dol– provider domain, in this case means computers on the local network of the Russian company Demos;
smtp– domain of the Demos server group serving the email system;
win– the name of a specific computer from the smtp group.
Thus, throughout the organization and internal structure, the DNS system resembles full path to a specific file in the tree of directories and files. One difference is that the higher-level domain in the DNS name is to the right. Just like an IP address, a DNS name must uniquely identify a computer on the Internet. The fully qualified domain name must end with a period.
ProtocolFrame Relay (FR).
Frame Relay is a protocol that describes the access interface to fast packet switching networks. It allows you to efficiently transmit extremely unevenly distributed traffic and ensures high speeds of information passing through the network, low latency times and rational use of bandwidth.
Over FR networks, it is possible to transmit not only data itself, but also digitized voice.
According to the seven-layer OSI open systems interaction model, FR is a second-layer protocol. However, it does not perform some of the functions required for protocols at this layer, but it does perform the functions of network layer protocols. At the same time, FR allows you to establish a connection over a network, which, according to OSI, is a function of layer 3 protocols.
Almost everyone today is familiar with the Internet first-hand. Internet in modern world plays an important role and is simply necessary for humanity. To the question of what a global network is, many people know the answer. At its core, a global network is an association of a large number of computers that can work remotely.
Currently, there are rules by which computers can communicate with each other. They can do all this exclusively with the help world wide web. So, what is the global Internet? The Internet assumes the same way for all computers to connect to the global network, uniform coding for data transmission and unified system data identification.
Why was the system created?
This system was created, first of all, so that people from different parts of the world have a constant opportunity to communicate with each other, and so that any interruptions in the system are not fatal. To understand what a global computer network is, it is enough to imagine a web. In the middle of the web, the weave is known to be denser - these are the so-called host computers, which constantly receive and send data that comes to them from different ends of the web. Since there are a huge number of host computers, neither viruses nor hacker attacks, nor power outages, will never be able to “put down” such a global network.
If any host computer suddenly stops working, then all data is automatically redirected to another host, so that all data remains safe. In the modern global network there are a huge number of host computers, and the number of ordinary user computers connected to them is even greater, although, probably, not a single ordinary person, going online, even imagines what part of global system he becomes.
A computer network is nothing more than the connection of several computers with each other, allowing them to exchange data. Networks are called global when they cover users from all over the world. Note that the first civilian computer networks appeared in the United States. But few people know that the very principle of this technology was first used in the USSR, and thanks to this, the first missile defense system was created a long time ago.
Today there is a fairly broad classification of networks. Based on territorial distribution, global, local and regional computer networks are distinguished. Global computer networks are networks located on the territory of a state or several states, for example, worldwide Internet network. cover an area of up to several tens of square meters, and regional ones are networks that are located in a region or city.
However, the two main terms in the classification of networks are: WAN and LAN.
To connect to such networks, modems were used that operated under the control of specialized telecommunications programs such as COMIT, PROCOM, BITCOM, MITEZ, etc. Controlled by MS-DOS, they exchanged information with the computer to which the connection was established.
The construction of a global network itself is not difficult to describe. The global network consists of cells, which are local networks. Local networks, in turn, consist of smaller networks and individual computers. It is this multi-level hierarchy that ensures the construction of the network. In addition, each device on the network must have its own IP address or unique identifier.
These days they are used less and less single computers included in global computer networks. Basically, these are home PCs. For the most part, network subscribers are those computers that are included in local computers; therefore, most often, specialists are exploring options on how best to ensure the interaction of several local computer networks at once. At the same time, it is necessary to ensure that the remote computer has a connection with any computer included in the remote local network, or vice versa. The last option is very relevant when the number of households and personal computers.
Nowadays, switching centers are used as network equipment, which are specialized devices for X.25 networks from manufacturers Telenet, Ericsson, Siemens, Alcatel, etc., but for networks with TCP/IP they use routers from Decnis and Cisco. However, technology does not stand still, and it is likely that in the future we will see an even more efficient organization of the global network, which today has a huge impact on the life of almost every person.
