Domain name Servers (DNS) are an important but invisible part of the internet, and form one of the largest databases on it. Each machine on an internet is assigned a unique address, called an IP address, which is 32 bit number and is expressed as 4 octets. The method user to represent these IP addresses is known as dotted decimal Notation”. A typical address looks like this: 199.249.150.4
It is very difficult to keep in mind the IP addresses of all the websites we visit daily, because it’s not easy to remember strings of numbers. However, we do remember words. This is where domain names come into the picture. If you want to connect to a particular site, you need to know its IP address but do need to know its URL. The DNS gets the mappings of the IP addresses and the corresponding names,
Names and numbers

DNS converts the machine names (such as www.xyz.com) to IP addresses (such as 199.249.150.9). Basically, it translates from a name to an address and from an address to a name.

The mapping from the IP address to the machine name is called reverse mapping .when you type http://www.xyz.com into your browser, the browser first needs to get the IP address of www.xyz.com. The machine uses a directory service to look up IP addresses and this service is called DNS. When you type www.xyz.com your machines firsts contacts a DNS server, asking it to find the IP address for www.xyz.com. This DNS server might then contact other DNS servers on the internet. DNS is therefore is considered as the global network of servers. The great advantage of DNS is that no organization is responsible for updating it. It is what is known as distributed database.