Introduction
The domain name system (DNS) is a global network of servers that translate host names like www.hkr.se into numerical IP (Internet Protocol) addresses, like 194.47.26.15, which computers on the internet use to communicate with each other.
The Internet Protocol address is a 32- bit integer. If somebody wants to send a message it is necessary to include the destination address, but people prefer to assign machines pronounceable, easily remembered names (host names). For this reason the Domain Name System is used. Without DNS, we'd all be memorizing long numbers instead of intuitive URLs or email addresses.
The Domain Name System (DNS) is a distributed database used by TCP/IP applications to map between hostnames and IP addresses, and to provide electronic mail routing information. Each site (university department or department within a company, for example) maintains its own database of information and runs a server program that other systems across the Internet can query. The DNS provides the protocol which allows clients and servers to communicate with each other.
The Structure Of Internet Host Names
The domain name system uses a hierarchical naming scheme known as domain names, which is similar to the Unix file system tree. The root of the DNS tree is a special node with a null label. The name of each node (except root) has to be up to 63 characters. The domain name of any node in the tree is the list of labels, starting at that node, working up to the root, using a period ("dot") to separate the labels (individual sections of a name might represent sites or a group, but the domain system simply calls each section a label ).
The difference between the Unix file system and the tree of the DNS is that in the DNS we start on the ground and "go up" till the root. Writing them in this order makes it possible to compress messages that contain multiple domain names. Thus, the domain name "hkr.se" contains two labels: "hkr" and "se". Any suffix of a label in a domain name is also called a domain. In the above example the lowest level domain is "hkr.se" (the domain name for the Kristianstad University in Sweden and the node "se" is the top-level node (after root). (Fig1 Hierarchical structure of the DNS)
Every node in the tree must have a unique domain name, but the same label can be used at different points in the tree. The top-level domains are divided into three areas:
1. arpa is a special domain used for address-to-name mapping.
