At the heart of the work global network The Internet is a set (stack) of TCP/IP protocols. But these terms seem complex only at first glance. In fact TCP/IP protocol stack is a simple set of rules for exchanging information, and these rules are actually well known to you, although you probably are not aware of it. Yes, that’s exactly how it is; essentially, there is nothing new in the principles underlying the TCP/IP protocols: everything new is well-forgotten old.

A person can learn in two ways:

1. Through stupid formal memorization of formulaic methods for solving standard problems (which is what is now mostly taught in school). Such training is ineffective. Surely you have seen the panic and complete helplessness of an accountant when changing the version office software- at the slightest change in the sequence of mouse clicks required to perform habitual actions. Or have you ever seen a person fall into a stupor when changing the desktop interface?
2. Through understanding the essence of problems, phenomena, patterns. Through understanding principles building this or that system. In this case, possession encyclopedic knowledge does not play a big role - the missing information is easy to find. The main thing is to know what to look for. And this requires not formal knowledge of the subject, but an understanding of the essence.

In this article, I propose to take the second path, since understanding the principles underlying the Internet will give you the opportunity to feel confident and free on the Internet - quickly solve problems that arise, correctly formulate problems and confidently communicate with technical support.

So, let's begin.

The operating principles of the TCP/IP Internet protocols are inherently very simple and strongly resemble the work of our Soviet postal service.

Remember how our regular mail works. First, you write a letter on a piece of paper, then put it in an envelope, seal it, back side envelope, write the addresses of the sender and recipient, and then take it to the nearest Postal office. Next, the letter passes through a chain of post offices to the nearest post office of the recipient, from where it is delivered by the postman to the specified address recipient and dropped into his mailbox (with his apartment number) or delivered personally. That's it, the letter has reached the recipient. When the recipient of the letter wants to answer you, he will swap the addresses of the recipient and the sender in his response letter, and the letter will be sent to you along the same chain, but in the opposite direction.

The envelope of the letter will read something like this:

Sender's address: From whom: Ivanov Ivan Ivanovich Where: Ivanteevka, st. Bolshaya, 8, apt. 25 Address of the recipient: To whom: Petrov Petr Petrovich Where: Moscow, Usachevsky lane, 105, apt. 110

Now we are ready to consider the interaction of computers and applications on the Internet (and in local network Same). Please note that the analogy with regular mail will be almost complete.

Each computer (aka: node, host) on the Internet also has a unique address, which is called an IP address ( Internet Protocol Address), for example: 195.34.32.116. An IP address consists of four decimal numbers(from 0 to 255), separated by a dot. But knowing only the IP address of the computer is not enough, because... Ultimately, it is not the computers themselves that exchange information, but the applications running on them. And several applications can run simultaneously on a computer (for example, a mail server, a web server, etc.). To deliver a regular paper letter, it is not enough to know only the address of the house - you also need to know the apartment number. Also every software application has a similar number called the port number. Most server applications have standard numbers, for example: Post service bound to port number 25 (they also say: “listens” to the port, receives messages on it), the web service is bound to port 80, FTP to port 21, and so on.

Thus we have the following practically complete analogy with our usual postal address:

"house address" = "computer IP" "apartment number" = "port number"

In computer networks operating using TCP/IP protocols, an analogue of a paper letter in an envelope is plastic bag, which contains the actual transmitted data and address information - the sender’s address and the recipient’s address, for example:

Source address: IP: 82.146.49.55 Port: 2049 Recipient address (Destination address): IP: 195.34.32.116 Port: 53 Package details: ...

Of course, the packages also contain service information, but this is not important for understanding the essence.

Please note the combination: "IP address and port number" - called "socket".

In our example, we send a packet from socket 82.146.49.55:2049 to socket 195.34.32.116:53, i.e. the packet will go to a computer with an IP address of 195.34.32.116, to port 53. And port 53 corresponds to a name recognition server (DNS server), which will receive this packet. Knowing the sender's address, this server will be able, after processing our request, to generate a response packet that will go in the opposite direction to the sender socket 82.146.49.55:2049, which for the DNS server will be the recipient socket.

As a rule, interaction is carried out according to the “client-server” scheme: the “client” requests some information (for example, a website page), the server accepts the request, processes it and sends the result. Server application port numbers are well known, for example: mail SMTP the server “listens” on port 25, the POP3 server that allows reading mail from your mailboxes “listens” on port 110, the web server listens on port 80, etc.

Most programs on home computer are clients - for example postal Outlook client, web browsers IE, FireFox, etc.

The port numbers on the client are not fixed like those on the server, but are assigned dynamically by the operating system. Fixed server ports usually have numbers up to 1024 (but there are exceptions), and client ports start after 1024.

Repetition is the mother of teaching: IP is the address of a computer (node, host) on the network, and port is the number specific application running on this computer.

However, it is difficult for a person to remember digital IP addresses - it is much more convenient to work with alphabetic names. After all, it is much easier to remember a word than a set of numbers. This is done - any digital IP address can be associated with an alphanumeric name. As a result, for example, instead of 82.146.49.55, you can use the name And the domain name service (DNS) (Domain Name System) handles the conversion of the domain name to a digital IP address.

Let's take a closer look at how this works. Your provider is clearly (on paper, for manual settings connections) or implicitly (via automatic setup connections) provides you with the IP address of the name server (DNS). On a computer with this IP address there is an application (name server) running that knows all the domain names on the Internet and their corresponding digital IP addresses. The DNS server “listens” to port 53, accepts requests to it and issues responses, for example:

Request from our computer: "What IP address corresponds to the name www.site?" Server response: "82.146.49.55."

Now let's look at what happens when you type in your browser Domain name(URL) of this site () and by clicking , in response from the web server you receive a page of this site.

For example:

IP address of our computer: 91.76.65.216 Browser: Internet Explorer (IE), DNS server (stream): 195.34.32.116 (yours may be different), The page we want to open: www.site.

Recruiting at address bar browser domain name and click . Next, the operating system performs approximately the following actions:

A request is sent (more precisely, a packet with a request) DNS server to socket 195.34.32.116:53. As discussed above, port 53 corresponds to the DNS server, an application that resolves names. And the DNS server, having processed our request, returns the IP address that matches the entered name.

The dialogue goes something like this:

What IP address corresponds to the name www.site? - 82.146.49.55 .

Next, our computer establishes a connection to the port 80 computer 82.146.49.55 and sends a request (request packet) to receive the page. Port 80 corresponds to the web server. Port 80 is usually not written in the address bar of the browser, because... is used by default, but it can also be specified explicitly after the colon - .

Having received a request from us, the web server processes it and sends us a page in several packets. HTML language- a text markup language that the browser understands.

Our browser, having received the page, displays it. As a result, we see on the screen home page this site.

Why do we need to understand these principles?

For example, you noticed strange behavior of your computer - strange network activity, slowdowns, etc. What to do? Open the console (click the “Start” button - “Run” - type cmd - “Ok”). In the console we type the command netstat -an and click . This utility will display a list of established connections between the sockets of our computer and the sockets of remote hosts. If we see some foreign IP addresses in the “External Address” column, and the 25th port after the colon, what could this mean? (Remember that port 25 corresponds to the mail server?) This means that your computer has established a connection with some mail server(servers) and sends some letters through it. And if your email client (Outlook for example) is not running at this time, and if there are still many such connections on port 25, then probably a virus has appeared on your computer that sends spam on your behalf or forwards your numbers credit cards along with passwords to attackers.

Also, an understanding of how the Internet works is necessary for correct settings firewall (in other words, firewall :)). This program (which often comes with an antivirus) is designed to filter packets - “friends” and “enemies”. Let your own people through, don’t let strangers in. For example, if your firewall tells you that someone wants to establish a connection to some port on your computer. Allow or deny?

And most importantly, this knowledge is extremely useful when communicating with technical support.

Finally, here is a list of ports that you are likely to encounter:

135-139 - these ports are used by Windows to access shared resources computer - folders, printers. Do not open these ports to the outside, i.e. to the regional local network and the Internet. They should be closed with a firewall. Also, if on the local network you do not see anything in network environment or they can’t see you, this is probably due to the fact that the firewall has blocked these ports. Thus, these ports must be open for the local network, but closed for the Internet. 21 - port FTP server. 25 - postal port SMTP server. Your email client sends letters through it. The IP address of the SMTP server and its port (25th) should be specified in the settings of your mail client. 110 - port POP3 server. Through it, your mail client picks up letters from your mailbox. The IP address of the POP3 server and its port (110th) should also be specified in the settings of your mail client. 80 - port WEB-servers. 3128, 8080 - proxy servers (configured in browser settings).

Several special IP addresses:

127.0.0.1 is the localhost address local system, i.e. local address your computer. 0.0.0.0 - this is how all IP addresses are designated. 192.168.xxx.xxx - addresses that can be used arbitrarily on local networks; they are not used on the global Internet. They are unique only within the local network. You can use addresses from this range at your discretion, for example, to build a home or office network.

What is the subnet mask and default gateway (router, router)?

(These parameters are set in the network connection settings).

It's simple. Computers are connected into local networks. On a local network, computers directly “see” only each other. Local networks are connected to each other through gateways (routers, routers). The subnet mask is designed to determine whether the recipient computer belongs to the same local network or not. If the receiving computer belongs to the same network as the sending computer, then the packet is sent to it directly, otherwise the packet is sent to the default gateway, which then, using routes known to it, transmits the packet to another network, i.e. to another post office (by analogy with the Soviet post office).

Finally, let’s look at what these unclear terms mean:

TCP/IP is the name of a set of network protocols. In fact, the transmitted packet goes through several layers. (Like in the post office: first you write a letter, then you put it in an addressed envelope, then the post office puts a stamp on it, etc.).

IP protocol is the protocol of the so-called network layer. The task of this level is to deliver IP packets from the sender's computer to the recipient's computer. In addition to the data itself, packets at this level have a source IP address and a recipient IP address. Port numbers are not used at the network layer. Which port, i.e. the application is addressed to this packet, whether this packet was delivered or was lost is unknown at this level - this is not its task, this is the task of the transport layer.

TCP and UDP These are protocols of the so-called transport layer. The transport layer sits above the network layer. At this level, a source port and a destination port are added to the packet.

TCP is a connection-oriented protocol with guaranteed packet delivery. First, special packets are exchanged to establish a connection, something like a handshake occurs (-Hello. -Hello. -Shall we chat? -Come on.). Then packets are sent back and forth over this connection (a conversation is in progress), and it is checked whether the packet has reached the recipient. If the packet is not received, it is sent again (“repeat, I didn’t hear”).

UDP is a connectionless protocol with non-guaranteed packet delivery. (Like: shouted something, but whether they heard you or not - it doesn’t matter).

Above the transport layer is the application layer. At this level, protocols such as http, ftp etc. For example, HTTP and FTP use the reliable TCP protocol, and the DNS server works through the unreliable UDP protocol.

How to view current connections?

Current connections can be viewed using the command

Netstat -an

(the n parameter specifies to display IP addresses instead of domain names).

This command runs like this:

“Start” - “Run” - type cmd - “Ok”. In the console that appears (black window), type the command netstat -an and click . The result will be a list of established connections between the sockets of our computer and remote nodes.

For example we get:

Active connections

Name	Local address	External address	State
TCP	0.0.0.0:135	0.0.0.0:0	LISTENING
TCP	91.76.65.216:139	0.0.0.0:0	LISTENING
TCP	91.76.65.216:1719	212.58.226.20:80	ESTABLISHED
TCP	91.76.65.216:1720	212.58.226.20:80	ESTABLISHED
TCP	91.76.65.216:1723	212.58.227.138:80	CLOSE_WAIT
TCP	91.76.65.216:1724	212.58.226.8:80	ESTABLISHED

...

In this example, 0.0.0.0:135 means that our computer listens (LISTENING) to port 135 at all its IP addresses and is ready to accept connections from anyone on it (0.0.0.0:0) via the TCP protocol.

91.76.65.216:139 - our computer listens to port 139 on its IP address 91.76.65.216.

The third line means that the connection is now established (ESTABLISHED) between our machine (91.76.65.216:1719) and the remote one (212.58.226.20:80). Port 80 means that our machine made a request to the web server (I actually have pages open in the browser).

In future articles we will look at how to apply this knowledge, e.g.

UNIX, which contributed to the growing popularity of the protocol, as manufacturers included TCP/IP in the software set of every UNIX computer. TCP/IP finds its mapping in the OSI reference model, as shown in Figure 3.1.

You can see that TCP/IP is located at layers three and four of the OSI model. The point of this is to leave the LAN technology to the developers. The purpose of TCP/IP is message transmission in local networks of any type and establishing communication using any network application.

The TCP/IP protocol works by being coupled to the OSI model at its two lowest layers—the data layer and the physical layer. This allows TCP/IP to find mutual language with virtually any network technology and, as a result, with any computer platform. TCP/IP includes four abstract layers, listed below.

Rice. 3.1.

• Network interface. Allows TCP/IP to actively interact with all modern network technologies based on the OSI model.
• Internetwork. Defines how IP controls forwarding messages through routers of a network space such as the Internet.
• Transport. Defines a mechanism for exchanging information between computers.
• Applied. Specifies network applications to perform tasks such as forwarding, Email and others.

Due to its widespread use, TCP/IP has become the de facto Internet standard. The computer on which it is implemented network technology, based on the OSI model (Ethernet or Token Ring), has the ability to communicate with other devices. In "Networking Fundamentals" we looked at layers 1 and 2 when discussing LAN technologies. Now we'll move on to OSI stack and see how the computer establishes communication on the Internet or on a private network. This section discusses the TCP/IP protocol and its configurations.

What is TCP/IP

The fact that computers can communicate with each other is itself a miracle. After all, these are computers from different manufacturers, working with various operating systems and protocols. Without some kind of common basis, such devices would not be able to exchange information. When sent over a network, data must be in a format that is understandable to both the sending device and the receiving device.

TCP/IP satisfies this condition through its internetworking layer. This layer directly matches the network layer of the OSI reference model and is based on a fixed message format called an IP datagram. A datagram is something like a basket in which all the information of a message is placed. For example, when you load a web page into a browser, what you see on the screen is delivered piecemeal by datagram.

It's easy to confuse datagrams with packets. A datagram is an information unit, while a packet is a physical message object (created at the third and higher layers) that is actually sent over the network. Although some consider these terms interchangeable, their distinction actually matters in a specific context - not here, of course. It is important to understand that the message is broken into fragments, transmitted over the network and reassembled at the receiving device.

The positive thing about this approach is that if a single packet is corrupted during transmission, then only that packet will need to be retransmitted, not the entire message. Another positive point is that no host has to wait indefinitely for another host's transmission to finish before sending its own message.

TCP and UDP

When sending an IP message over a network, one of the transport protocols is used: TCP or UDP. TCP (Transmission Control Protocol) makes up the first half of the acronym TCP/IP. The User Datagram Protocol (UDP) is used instead of TCP to transport less than important messages. Both protocols are used for the correct exchange of messages in TCP/IP networks. There is one significant difference between these protocols.

TCP is called a reliable protocol because it communicates with the recipient to verify that the message was received.

UDP is called an unreliable protocol because it does not even attempt to contact the recipient to verify delivery.

It is important to remember that only one protocol can be used to deliver a message. For example, when a web page is loaded, packet delivery is controlled by TCP without any UDP intervention. On the other hand, Trivial File Transfer Protocol File Transfer Protocol, TFTP) downloads or sends messages under the control of the UDP protocol.

The transport method used depends on the application - it could be email, HTTP, the application responsible for networking work, and so on. Developers network programs use UDP wherever possible, since this protocol reduces excess traffic. TCP protocol attaches more effort for guaranteed delivery and transmits many more packets than UDP. Figure 3.2 provides a list of network applications and shows which applications use TCP and which use UDP. For example, FTP and TFTP do essentially the same thing. However, TFTP is mainly used for downloading and copying programs. network devices. TFTP can use UDP because if the message fails to be delivered, nothing bad happens because the message was not intended for the end user, but for the network administrator, whose priority level is much lower. Another example is a voice video session, in which ports for both TCP and UDP sessions can be used. Thus, a TCP session is initiated to exchange data when a telephone connection is established, while the TCP session itself phone conversation transmitted via UDP. This is due to the speed of voice and video streaming. If a packet is lost, there is no point in resending it, since it will no longer match the data flow.

Rice. 3.2.

IP Datagram Format

IP packets can be broken down into datagrams. The datagram format creates fields for the payload and for message transmission control data. Figure 3.3 shows the datagram diagram.

Note. Don't be fooled by the size of the data field in a datagram. The datagram is not overloaded with additional data. The data field is actually the largest field in the datagram.

Rice. 3.3.

It is important to remember that IP packets can have different lengths. In "Networking Fundamentals" it was said that information packets in Ethernet networks have a size from 64 to 1400 bytes. In the Token Ring network their length is 4000 bytes, in ATM networks- 53 bytes.

Note. The use of bytes in a datagram can be confusing, since data transfer is often associated with concepts such as megabits and gigabits per second. However, because computers prefer to work with data bytes, datagrams also use bytes.

If you look again at the datagram format in Figure 3.3, you'll notice that the leftmost margins are a constant value. This happens because CPU A person working with packets must know where each field begins. Without standardization of these fields, the final bits will be a jumble of ones and zeros. On the right side of the datagram are packets of variable length. The purpose of the various fields in a datagram is as follows.

• VER. The version of the IP protocol used by the station where the original message appeared. Current version IP is version 4. This field ensures concurrent existence different versions in the internetwork space.
• HLEN. The field informs the receiving device of the length of the header so that the CPU knows where the data field begins.
• Service type. Code that tells the router the type of packet control in terms of service level (reliability, priority, deferment, etc.).
• Length. The total number of bytes in the packet, including header fields and data fields.
• ID, frags and frags offset. These fields tell the router how to fragment and reassemble the packet and how to compensate for differences in frame size that may occur as the packet traverses LAN segments with different network technologies (Ethernet, FDDI, etc.).
• TTL. An abbreviation for Time to Live is a number that decreases by one each time a packet is sent. If the lifetime becomes zero, the packet ceases to exist. TTL prevents loops and lost packets from wandering endlessly across the Internet.
• Protocol. The transport protocol to use to transmit the packet. The most common protocol specified in this field is TCP, but other protocols may be used.
• Header checksum. A checksum is a number that is used to verify the integrity of a message. If the checksums of all message packets do not match correct value, this means that the message has been corrupted.
• Source IP address. The 32-bit address of the host that sent the message (usually a personal computer or server).
• Destination IP address. The 32-bit address of the host to which the message was sent (usually a personal computer or server).
• IP options. Used for network testing or other special purposes.
• Padding. Fills all unused (empty) bit positions so that the processor can correctly determine the position of the first bit in the data field.
• Data. The payload of the sent message. For example, the package data field may contain the text of an email.

As mentioned earlier, the packet consists of two main components: data about message processing, located in the header, and the information itself. The information part is located in the payload sector. You can imagine this sector as the cargo compartment of a spaceship. The title is everything on-board computers shuttle in the control cabin. It manages all the information needed by all the different routers and computers along the message path, and is used to maintain a certain order in assembling the message from individual packets.

The Internet, which is a network of networks and unites a huge number of different local, regional and corporate networks, operates and develops through the use of a single TCP/IP data transfer protocol. The term TCP/IP includes the name of two protocols:

Transmission Control Protocol (TCP) - transport protocol;

Internet Protocol (IP) is a routing protocol.

Routing protocol. The IP protocol ensures the transfer of information between computers on a network. Let's consider the operation of this protocol by analogy with the transfer of information using regular mail. In order for the letter to reach its intended destination, the address of the recipient (who the letter is to) and the address of the sender (from whom the letter is from) are indicated on the envelope.

Similarly, information transmitted over the network is “packed in an envelope” on which the IP addresses of the recipient’s and sender’s computers are “written”, for example “To: 198.78.213.185”, “From: 193.124.5.33”. Contents of the envelope on computer language called an IP packet and is a set of bytes.

In the process of forwarding regular letters, they are first delivered to the post office closest to the sender, and then transferred along the chain of post offices to the post office closest to the recipient. At intermediate post offices, letters are sorted, that is, it is determined to which next post office a particular letter should be sent.

On the way to the recipient computer, IP packets also pass through numerous intermediate Internet servers where the routing operation is performed. As a result of routing, IP packets are sent from one Internet server to another, gradually approaching the recipient computer.

Internet Protocol (IP) provides routing of IP packets, that is, the delivery of information from the sending computer to the receiving computer.

Determining the route for information to pass through. The “geography” of the Internet differs significantly from the geography we are accustomed to. The speed of obtaining information does not depend on the distance of the Web server, but on the number of intermediate servers and the quality of communication lines (their capacity) through which information is transmitted from node to node.

You can get acquainted with the route of information on the Internet quite simply. The special program tracert.exe, which is included in Windows composition, allows you to track through which servers and with what delay information is transferred from the selected Internet server to your computer.

Let's see how access to information is implemented in the "Moscow" part of the Internet to one of the most popular search servers Russian Internet www.rambler.ru.

Determining the route of information passage

1. Connect to the Internet, enter the command [Programs-MS-DOS Session].

2. In the MS-DOS Session window, in response to the system prompt, enter the command.

3. After some time, a trace of information transfer will appear, that is, a list of nodes through which information is transmitted to your computer, and the time of transmission between nodes.

Route tracing transmission of information shows that the server www.rambler.ru is located at a “distance” of 7 transitions from us, i.e. information is transmitted through six intermediate Internet servers (through the servers of the Moscow providers MTU-Inform and Demos). The speed of information transfer between nodes is quite high; one “transition” takes from 126 to 138 ms.

Transport protocol. Now let’s imagine that we need to send a multi-page manuscript by mail, but the post office does not accept parcels or parcels. The idea is simple: if the manuscript does not fit into a regular postal envelope, it must be disassembled into sheets and sent in several envelopes. In this case, the sheets of the manuscript must be numbered so that the recipient knows in what sequence these sheets will be combined later.

A similar situation often occurs on the Internet when computers exchange large files. If you send such a file as a whole, it can “clog” the communication channel for a long time, making it inaccessible for sending other messages.

To prevent this from happening, the sending computer must be set to large file into small parts, number them and transport them in separate IP packets to the recipient computer. On the recipient computer you need to collect original file from individual parts in the correct sequence.

Transmission Control Protocol (TCP), that is, a transport protocol, ensures that files are split into IP packets during transmission and files are assembled during reception.

Determining the time of IP packet exchange. Time of exchange of IP packets between local computer and the Internet server can be determined using the ping utility, which is included in operating system Windows. The utility sends four IP packets to the specified address and shows the total transmission and reception time for each packet

73. Searching for information on the Internet.

There is a largely fair opinion that today the Internet “has everything” and the only problem is how to find the necessary information. Herself open architecture The network is facilitated by the fact that it lacks any centralization and the most valuable data for you, which you have unsuccessfully searched all over the world, may turn out to be located on a server in the same city as you. There are two complementary approaches to collecting information about Internet resources: creating indexes And creating directories:

With the first method, powerful search servers continuously “search” the Internet, creating and expanding Database, containing information about which documents on the Network contain certain keywords. Thus, in reality the search takes place not on Internet servers, which would be technically impossible, but on the database of a search engine, 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.

In the second case, the server is organized as library catalog, containing a hierarchy of sections and subsections that store links to documents that correspond to the topic of the subsection. 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 size of the directory is usually less than the number of sites indexed by the search engine.

Working with search servers. When entering the main page of the search server, just type your query in the input field in the form of a set of keywords and press Enter or the start search button.

Queries can contain any words, and it is not necessary to worry about cases and declensions - for example, the queries “philosophy essay” and “philosophy essay” are quite correct.

Modern search servers understand natural language quite well, however, many of them retain advanced or special search capabilities that allow you to search for words by mask, combine query words with logical operations “AND”, “OR”, etc.

After completing the database search, the server displays the first batch of 10 or more documents containing the keywords. In addition to the link, there are usually several lines of text describing the document or just its beginning. By opening links in a new or the same browser window, you can move to the selected documents, and the line of links at the bottom of the page allows you to move to the next portion of documents. This line looks something like this:

Different servers sort found documents in different ways - by date of creation, by document traffic, by the presence of all or part of the query words in the document ( relevance), some servers allow you to narrow your search by selecting the category of the document you are looking for on the main page - for example, the query “banks” in the “business world” category is unlikely to find information about cans.

Among the popular Russian-language search tools we can name servers Yandex, Aport And Rambler, indexing tens of thousands of servers and tens of millions of documents. Popular from foreign servers Altavista, Excite, Hotbot, Lycos, WebCrawler, OpenText.

Finally, there are many pages on the Internet for metasearch, allowing you to access several popular search servers at once with the same query - look, for example, at the pages http://www.find.ru/ or http://www.rinet.ru/buki/.

Working with resource catalogs. When we enter the main page of the catalog, we find ourselves in an extensive menu or table for selecting categories, each of which can contain nested subcategories. There is no standard here, but still the directory structures are very similar, everywhere you can find sections “business” or “business world”, “computers”, “programming” or “Internet”, “humor” or “hobbies”, etc. . By moving through categories, you can get to links to specific documents, which, just like on a search server, are issued in portions and are accompanied by brief information.

Today there are many large directories with tens of thousands of links, from domestic directories we can name http://www.list.ru/, http://www.weblist.ru/, http://www.stars.ru/, http://www.au.ru/, http://www.ru/, http://www.ulitka.ru/, and from foreign ones - Yahoo, Magellan.

Often the catalog also has a form for searching by keywords among the documents listed in it.

Search rules. Some simple tips related to Internet searches.

clearly define in advance the topic of your search, keywords and the time you are willing to spend on this search; select a search server - it is useful to store links to the best of them in Favorites;

do not be afraid natural language, but check the correct spelling of words, for example, when Microsoft help Word;

Use capital letters only in names and titles. Many search engines will correctly process the query “abstract”, but not “Abstract”;

Interaction between computers on the Internet is carried out through network protocols, which are an agreed upon set of specific rules, according to which different devices data transmissions exchange information. There are protocols for error control formats and other types of protocols. The most commonly used protocol in global internetworking is TCP-IP.

What kind of technology is this? The name TCP-IP comes from two network protocols: TCP and IP. Of course, the construction of networks is not limited to these two protocols, but they are basic as far as the organization of data transmission is concerned. In fact, TCP-IP is a set of protocols that allows individual networks to come together to form

The TCP-IP protocol, which cannot be described only by the definitions of IP and TCP, also includes the protocols UDP, SMTP, ICMP, FTP, telnet, and more. These and other TCP-IP protocols provide the most complete operation of the Internet.

Below we provide a detailed description of each protocol included in general concept TCP-IP.

. Internet protocol(IP) is responsible for the direct transmission of information on the network. The information is divided into parts (in other words, packets) and transmitted to the recipient from the sender. For accurate addressing, you need to specify the exact address or coordinates of the recipient. Such addresses consist of four bytes, which are separated from each other by dots. Each computer's address is unique.

However, using the IP protocol alone may not be enough for correct data transmission, since the volume of most of the transmitted information is more than 1500 characters, which no longer fits into one packet, and some packets may be lost during transmission or sent in the wrong order, what is needed.

. Transmission Control Protocol(TCP) is used for more high level than the previous one. Based on the IP protocol's ability to carry information from one host to another, the TCP protocol allows large amounts of information to be sent. TCP is also responsible for separation transmitted information into separate parts - packets - and correct recovery of data from packets received after transmission. Wherein this protocol automatically retransmits packets that contain errors.

Management of the organization of data transfer in large volumes can be carried out using a number of protocols that have special functional purpose. In particular, there are the following types of TCP protocols.

1. FTP(File Transfer Protocol) organizes file transfer and is used to transfer information between two Internet nodes using TCP connections in the form of a binary or simple text file, as a named area in computer memory. In this case, it does not matter where these nodes are located and how they are connected to each other.

2. User Datagram Protocol, or User Datagram Protocol, is connection independent and transmits data in packets called UDP datagrams. However, this protocol is not as reliable as TCP because the sender does not know whether the packet was actually received.

3. ICMP(Internet Control Message Protocol) exists to transmit error messages that occur during data exchange on the Internet. However, the ICMP protocol only reports errors, but does not eliminate the reasons that led to these errors.

4. Telnet- which is used to implement a text interface on a network using the TCP transport.

5. SMTP(Simple Mail Transfer Protocol) is a special by email, which defines the format of messages that are sent from one computer, called an SMTP client, to another computer running an SMTP server. In this case, this transfer can be delayed for some time until the work of both the client and the server is activated.

Data transmission scheme via TCP-IP protocol

1. The TCP protocol breaks the entire amount of data into packets and numbers them, packing them into TCP envelopes, which allows you to restore the order in which parts of information are received. When data is placed in such an envelope, a calculation occurs checksum, which is then written to the TCP header.

3. TCP then checks to see if all packets have been received. If, during reception, the newly calculated one does not coincide with that indicated on the envelope, this indicates that some of the information was lost or distorted during transmission, the TCP-IP protocol again requests the forwarding of this packet. Confirmation of the receipt of data from the recipient is also required.

4. After confirming the receipt of all packets, the TCP protocol orders them accordingly and reassembles them into a single whole.

The TCP protocol uses repeated data transmissions and waiting periods (or timeouts) to ensure reliable delivery of information. Packets can be transmitted in two directions simultaneously.

Thus, TCP-IP eliminates the need for retransmissions and waits for application processes (such as Telnet and FTP).

The TCP/IP protocol stack is the alpha and omega of the Internet, and you need to not only know, but also understand the model and operating principle of the stack.

We figured out the classification, network standards and the OSI model. Now let's talk about the stack on which it is built world system United computer networks Internet.

TCP/IP model

Initially, this stack was created to combine large computers at universities telephone lines point-to-point communications. But when new technologies appeared, broadcast (Ethernet) and satellite, it became necessary to adapt TCP/IP, which turned out to be a difficult task. That is why, along with OSI, the TCP/IP model appeared.

The model describes how it is necessary to build networks based on various technologies in order for the TCP/IP protocol stack to work in them.

The table shows a comparison of OSI and TCP/IP models. The latter includes 4 levels:

1. The lowest one, level network interfaces , provides interaction with network technologies (Ethernet, Wi-Fi, etc.). This is a combination of the functions of channel and physical levels OSI.
2. Internet level stands higher and has similar tasks to the network layer of the OSI model. It provides search for the optimal route, including identifying network problems. It is at this level that the router operates.
3. Transport responsible for communication between processes on different computers, as well as for the delivery of transmitted information without duplication, loss or error, in the required sequence.
4. Applied combines 3 layers of the OSI model: session, presentation and application. That is, it performs functions such as session support, protocol and information conversion, and user-network interaction.

Sometimes experts try to combine both models into something common. For example, below is a five-level representation of symbiosis from the authors of Computer Networks E. Tanenbaum and D. Weatherall:

The OSI model has good theoretical development, but the protocols are not used. The TCP/IP model is different: the protocols are widely used, but the model is only suitable for describing TCP/IP-based networks.

Don't confuse them:

• TCP/IP is a protocol stack that forms the basis of the Internet.
• OSI Model (Basic Reference Model Interactions Open Systems) is suitable for describing a wide variety of networks.

TCP/IP protocol stack

Let's look at each level in more detail.

The lower level of network interfaces includes Ethernet, Wi-Fi and DSL (modem). Data network technologies They are not formally part of the stack, but are extremely important in the operation of the Internet as a whole.

The main network layer protocol is IP (Internet Protocol). It is a routed protocol, part of which is network addressing (IP address). Additional protocols such as ICMP, ARRP and DHCP also work here. They keep networks running.

At the transport level there is TCP, a protocol that ensures data transfer with a delivery guarantee, and UDP, a protocol for fast transfer data, but without guarantee.

The application layer is HTTP (for the web), SMTP (mail transfer), DNS (assigning friendly domain names to IP addresses), FTP (file transfer). There are more protocols at the application level of the TCP/IP stack, but the ones listed can be called the most significant for consideration.

Remember that the TCP/IP protocol stack defines the standards for communication between devices and contains internetworking and routing conventions.