What is DNS - Domain Name System?
A Domain Name System or DNS can be considered to be the particular source of information (or website) available on the Internet. The most important function of DNS is to provide an interface through which plaintext words which can be understood by human beings i.e. the web address of a particular site is translated into a set of numbers readable by the computer.

DNS is also used to store a lot of information such as lists of mail servers which will accept electronic mails, for a particular domain. DNS is a highly essential component for internet usage as it provides universal redirection service which is based on keyboard operations.

The DNS space consists of a tree of domain names. One or more resource records are a part of a node or a leaf which holds information associated with the domain name. This DNS domain name tree is subdivided into a number of zones. A zone is nothing other than a set of nodes connected together and served by a common DNS nameserver. This process is however authoritative.
A resolver is used to look up the information associated with all the nodes. A resolver communicates with a nameserver by sending it various DNS requests, and then analyzing the DNS responses it receives from the nameserver. Resolving is an iterative process which involves looping through multiple DNS nameservers in order to obtain the required information.

DNS is a collection of multiple DNS servers. Each domain and subdomain will have one or more authoritative dedicated DNS servers which possess all information about that domain. The hierarchy of the domains and subdomains is reflected in the hierarchy of the Domain Name System Servers. At the head of the hierarchy tree is the root nameserver. A root nameserver is the server which is queried by the resolver.

The facility of local caching is also available in the DNS resolution process. Caching is the recording of the results of a DNS query for a certain amount of time when a successful answer is obtained. The resolver takes a certain amount of time to cache each DNS response. This time is known as the Time To Live (TTL)an it is the duration a DNS response will remain valid. Such a TTL may range from a few seconds to several days and in very rare cases even weeks.

The DNS is also used for other applications such as :-
Hostnames and IP addresses do not always match on a one-to-one basis. Several hostnames may correspond to a single IP address and may be combined with virtual hosting, this allows a single machine to serve many web sites. In an alternative basis a single hostname may correspond to many IP addresses.
 

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