To identify an entity, TCP/IP protocols use the IP address, which uniquely identifies the connection of a host to the Internet. However, people prefer to use names instead of addresses. The naming scheme used in the Internet is called the Domain Name System (DNS). Syntactically, each computer name consists of a sequence of alpha­numeric segments separated by periods. For example, a computer in the Information Technology Department at Vellore Institute of Technology has the domain name:
mordred.it.vit.edu
While a computer in the Vellore Engineering College at Madras University has the domain name:
www.vec.madrasuni.edu
Domain names are hierarchical with the most significant part of the name on the right. The left-most segment of' a name (mordred and www in the examples) is the name of an individual computer. Other segments in a domain name identify the group that owns the name. For example, the segments vit and madrasuni each give the name of the university.
The domain name system does specify values for the most significant segment, which is called the top-level of the DNS. Table 12.1 lists the possible top-level domains.

Seven additional top-level domains were proposed in 1997 to further divide the namespace and eliminate the overcrowding which has occurred in the commercial domain. The proposed names include firm, store, web, arts, rec, info and nom. Arts and rec were proposed to accommodate organizations such as art museums and recreational web sites; while nom was proposed to permit individuals to register their names. For many reasons, however, the new names have not received official approval.
When an organization wants to participate in the domain name system, the organization must apply for a name under one of the existing top-level domains. Most corporations choose to register under the com domain. For example, a corporation named "Videsh Sanchar Nigam Limited" might request to be assigned the domain vsnl under the top-level domain com. If it approves the request, the Internet authority responsible for domain names will assign the domain:
vsnl.com
Once an organization has been assigned a domain, the suffix is reserved for the organization: no other organization will be assigned the same name suffix. For example, once vsnl.com has been assigned, another organization cannot apply for vsnl.com, it is possible to apply for vsnl.edu or vsnl.org etc. To summarize:
To obtain a domain, an organization must register with the Internet authority. A unique domain suffix is assigned to each organization.
  Country Domains
The country domain section follows the same format as the generic domains but uses two-character country abbreviations (for example, in for India) in place of three character organizational abbreviations at the first level. Second-level labels can be organizational or they can be more specific, national designations. The United States, for example, uses state abbreviation of us (for example, ca.us).
Some foreign countries have adopted a combination of geographic and organizational domain names. For example, universities in the United Kingdom and in India register under the domains:
ac.uk ac.in
where ac is an abbreviation for academic and uk and in are the official country codes for the United Kingdom and India.
  The DNS Client-Server Model
One of the main features of the Domain Naming System is autonomy-the system is designed to allow each organization to assign names to computers or to change those names without informing a central authority. The naming hierarchy helps achieve autonomy by allowing an organization to control all names with a particular suffix. Thus, Videsh Sanchar Nigam Limited is free to create or change any name that ends with vsnl.com, while IBM corporation is free to create or change names that end with ibm.com.
In addition to hierarchical names, the DNS uses client-server interaction to aid autonomy. In essence, the entire naming system operates as a large, distributed database. Most organizations that have an Internet connection run a domain name server. Each server contains information that links the server to other domain name servers; the resulting set of servers functions as a large, coordinated database of names. Whenever an application needs to translate a name to an IP address, the application becomes a client of the naming system. The client places the name to be translated in a DNS request message and sends the request to a DNS server. The server extracts the name from the request, translates the name to an equivalent IP address, and returns the resulting address to the application in a reply message.

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