The Domain Names Service is intended that the PC working in Internet, could learn on a domain name the IP-address the PC necessary to them, and also some other information; and under IP-number could learn a domain name of the PC.

The general control and coordination over system of domain names, including development of rules and conditions of addition of new domains of top level, behind address space IP, behind purpose of technical parameters of the Internet is carried out by noncommercial public corporation ICANN (The Internet Corporation for Assigned Names and Numbers), registered in California in 1998.

The service of Domain Names has been developed for named PC in a global network. The basic feature of a global network the distributed administration when one manager physically cannot follow allocation of names is. Therefore the Domain Names Service functions on a principle of delegation of powers. Each machine or knows the answer on the question, or knows whom to ask. At correct functioning the system is closed, if the requested information is available for someone it will be found and informed to the client, or if the question has no the answer, the client will receive the message about impossibility of reception of the answer to a question.

Each client knows the server. Usually is underlined some servers - if the first do not answer, the client addresses to second and so on before exhaustion of the list. Basically it is unimportant, to what server to address - they give (should to give at correct functioning) identical answers to any inquiry. Therefore for acceleration of work usually specify the nearest. It is necessary to remember, that by one machine can simultaneously function a name-server and programs-clients; therefore if by the PC the name-server as a name-server on it I should be registered is started.

There is the certain domain of top level designated by a point: ".". There are nine servers (at least on mine name-server it is written down so much) which are responsible for this zone. They do not know any domain name - only will authorize servers of the top zones. Servers of the top zones too shun to store the information on concrete machines and transfer this right to underlaying servers. Here already there are first mentions of concrete machines, no less than occurs authorization of underlaying servers.

Any server with a domain name from one segment is unknown; domain names from two are very seldom used segments; names make overwhelming share of all names Internet of three and four segments; names from five segments meet seldom enough, and from six and more is not present.

Let's admit, the client has requested "www.organizaition.sity.country address. Information search on a domain name occurs the following шn the image:

It is the approached model which nevertheless allows to present work of system DNS.

However this harmonous picture is deformed by systems cashing and secondary servers. The matter is that having received the answer to the the question, DNS-server receives also some number which speaks it about after what time this information should be considered become outdated. Thus, all the servers participated in search of the answer to a question, set by the client, can (and most likely will be) to remember both the answer to an asked question, and a way on which there was a search. At the following the inquiries having the general right part with recently made inquiries, search will be simplified accelerated).

Besides the majority of zones has secondary servers which contain copies of data from primary servers. A server overlying the inquiry both can direct zones to a primary server, and any of secondary, being based on the reasons about that, what of them is closer.

I wish to pay special attention on similarity, distinction and interaction of systems DNS and IP-routings. As well as IP-routing, DNS works by a principle of delegation of powers, but allocation of domain names does not depend at all on allocation of IP-addresses. For an example we shall consider the domain freebsd.org. It is the domain of the organization, an operational system engaged by distribution FreeBSD Unix. The FTP-server containing the distribution kit of operational system and set of utilities for it, has copies in several tens countries. Names of servers look so:

• ftp.freebsd.org - a primary server in the USA
• ftp.country.freebsd.org - the basic server in the country
• ftpnumber.countri.freebsd.org - an additional server in the country

However, some services of it have not enough - so E-mail demands, that the PC accepting the letter, has recognized as the the address, specified as destination. Report HTTP 1.1 (in 1.0 it was not) demands, that in HTTP-inquiry it was specified not a way to a file, counted from a root of a server (though such inquiries too admit), but also a name of a server; thus a server knows, what names - it, and the others обрезает and serves according to HTTP 1.0.

Delegation of a zone... in-addr.arpa it is given only from the provider together with IP-addresses. Actually, it is connected with applicability ReverceDNS - to inform a domain name to the IP-address. For certain the master of a zone freebsd.org holds a Reverce-zone for IP-numbers, allocated to university Berkley; but all these servers (except for a server located at university) do not enter in this Reverce-zone, so, not control to it.

One of problems that it is possible to allocate a Reverce-zone only for a network of class A, B or C (on 16777216, 65536 or 256 addresses) and in any way differently. It is possible to receive the rights to some zones of one or different classes but what to do by that to whom have allocated it is less than 256 addresses? And in fact in conditions of exhaustion of address space not a rarity of allocation of a pool already on 16 addresses!

As a rule, the provider gives to the client the whole complex of services. Into number of rendered DNS-services enter:

• Delegation of a zone... in-addr.arpa to the clients having a pool of addresses, multiple 256.
• Registration of a domain name of the client at the holder of that zone in which the client wishes to be registered;
• Maintenance of a secondary server of direct and return DNS-zones of the client;
• Maintenance of a primary server of these zones if the client for any reason does not support their itself (especially it concerns to a case of virtual zones and to a case of allocation of a small pool of addresses);