Modern internet users got used to symbolic site addresses, e.g. test.com. Indeed such addresses are easier to type and to remember. Symbolic addresses actually function thanks to DNS or Domain Name System which operates in such close companionship with the Internet that the absolute majority of users don’t even give a thought about its existence. And a certain number of “experienced” users think of DNS only when there arise problems with it.
Meanwhile, in order to assign addresses to internet devices special numerical codes are used that are called IP-addresses. Domain Name System serves to translate symbolic addresses into numerical ones. A traditional IP-address is made up of four numbers in the decimal system, e.g. 192.168.175.13 or 220.127.116.11. DNS gives an opportunity to match a numerical IP-address and a symbolic one, for example, 18.104.22.168 = test.com.
At the same time a symbolic address in DNS is a character string composed according to certain rules. The most important rule here is a domain name hierarchy. DNS has a tree structure. Elements of this structure are called domains. Each domain may contain a multitude of sub-domains.
The DNS tree is traditionally subdivided into levels: the first level, the second one, the third one, etc. The system itself starts from the one and only root domain (zero level). It’s interesting but nowadays only specialists remember about the existence of this root domain due to the fact that the modern DNS allows not to indicate it in the address bar. An address bar with the root domain name reads for example like this: «site.test.com.» — here the root domain is separated by the last dot on the right.
It’s easy to guess that DNS addresses are written in the form of a sequence reflecting the name hierarchy. The higher the domain level is – the more to the right it is written in the address bar. Domains are separated by dots. Let’s take the following address as an example: www.subdomain.test.com. In here ‘www’ is a domain of the fourth level, while the rest of the domains mentioned in the address are located within the first level domain ‘COM’. For instance, subdomain.test.com is a third level domain. It’s crucial to understand that a usual website address, e.g. www.test.com, is a third level domain (www), located inside the second level domain test.com.
DNS uses a distributed system of special servers in order to translate domain names and IP-addresses. Each server takes care of its “set of clients” by translating addresses for them. DNS servers have the hierarchy of ‘trust’ and they practise distribution of ‘responsibility zones’, which means that a server may be responsible for a certain set of domains. At the same time DNS servers being part of the global DNS Internet system are interconnected and they exchange information using quite complicated protocols. For example, DNS servers can share information with each other about changes of addresses in a certain domain zone. Everything mentioned above is directed at providing successful translation of all addresses getting to DNS from any computer with the Internet connection, wherever this computer may be located. The so-called DNS root servers are the most important ones as they provide and are responsible for the functioning of the whole Internet domain name system. There exist 13 such servers and all of them belong to the ICANN technical center. Root servers of the first level domains (e.g. COM) are also key elements because they provide distribution of DNS-information about the domain located within their responsibility zone.
From a user’s perspective and in a very simplified manner the way the DNS searches for website addresses may be described as follows. When a user types in a website address in the browser address bar, let’s say subdomain.test.com, the computer makes a query to this or that DNS server familiar to this computer, asking the server which IP-address is connected with the domain address requested by the user. As a reply the DNS server sends the required IP-address, having verified the correspondence using its own internal tables or having made a query to other DNS servers. Then the browser gets connected with the website through the received IP-address.
The Domain Name System was developed in the 1980-s and since then has been successfully providing comfort when dealing with the Internet address issues.