The unique name that identifies an Internet site. Domain Names always have 2 or more parts, separated by dots. The part on the left is the most specific, and the part on the right is the most general.A given machine may have more than one Domain Name but a given Domain Name points to only one machine. For example, the domain names: yahoo.com mail.yahoo.com workshop.yahoo.com can all refer to the same machine, but each domain name can refer to no more than one machine. Usually, all of the machines on a given Network will have the same thing as the right-hand portion of their Domain Names.It is also possible for a Domain Name to exist but not be connected to an actual machine. This is often done so that a group or business can have an Internet e-mail address without having to establish a real Internet site. In these cases, some real Internet machine must handle the mail on behalf of the listed Domain Name.

Domain name system is a hierarchical system where we have a top level domain serving sub domain and clients with names & ip address.The system that runs the name services to resolve names into ipaddresses is called name server and the sofware is generally BIND (Berkley Internet Domain).Core process of DNS is a daemon called named . Depending on the role assigned the name servers can be a primary, secondry or caching only. Secondry server takes over when primary is down and is updated automatically . Caching server provide only the caching information to the clients .Each of domain or sub domain have information (in zone files or data files) about its clients and is called authorative for these clients . For other clients for which it doesn't have any information or it is not authorative , it passes query to its higher domain .The client knows about their name servers through a file called resolve.conf which contains addresses of the name servers (Primary secondary and Caching) along with their domain name.The main files in serve are named.conf which contains server parameters and reference to other data files containing client information.

The Domain Name System (DNS) is basically a large database which resides on various computers and it contains the names and IP addresses of various hosts on the internet and various domains. The Domain Name System is used to provide information to the Domain Name Service to use when queries are made. The service is the act of querying the database, and the system is the data structure and data itself. The Domain Name System is similar to a file system in Unix or DOS starting with a root. Branches attach to the root to create a huge set of paths. Each branch in the DNS is called a label. Each label can be 63 characters long, but most are less. Each text word between the dots can be 63 characters in length, with the total domain name (all the labels) limited to 255 bytes in overall length. The domain name system database is divided into sections called zones. The name servers in their respective zones are responsible for answering queries for their zones. A zone is a subtree of DNS and is administered separately. There are multiple name servers for a zone. There is usually one primary nameserver and one or more secondary name servers. A name server may be authoritative for more than one zone.
DNS names are assigned through the Internet Registries by the Internet Assigned Number Authority (IANA). The domain name is a name assigned to an internet domain. For example, mycollege.edu represents the domain name of an educational institution. The names microsoft.com and 3Com.com represent the domain names at those commercial companies. Naming hosts within the domain is up to individuals administer their domain.
Access to the Domain name database is through a resolver which may be a program or part of an operating system that resides on users workstations. In Unix the resolver is accessed by using the library functions "gethostbyname" and "gethostbyaddr". The resolver will send requests to the name servers to return information requested by the user. The requesting computer tries to connect to the name server using its IP address rather than the name.

There are two types of queries issued: