Domain name Servers
Domain name Servers (DNS) are an important but invisible part of internet, and form one of 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: 18.104.22.168
It is very difficult to keep in mind IP addresses of all 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 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 mappings of IP addresses and corresponding names.
Names and numbers
DNS converts machine names (such as www.xyz.com) to IP addresses (such as 22.214.171.124). Basically, it translates from a name to an address and from an address to a name.
The mapping from IP address to machine name is called reverse mapping .when you type http://www.xyz.com into your browser, browser first needs to get 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 IP address for www.xyz.com. This DNS server might then contact other DNS servers on internet. DNS is therefore is considered as 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.
The three letter codes
A DNS server is just a computer that's running DNS software. The most popular DNS software is BIND (Berkeley Internet Name Domain) DNS is hierarchical, tree-structured system. The top is donated by'.'. And is known as root of system. Below root there are seven immediate sub domain nodes and these are ‘com', ‘org', ‘gov', ‘mil', ‘net', ‘edu', ‘Int', etc.
DNS consists of two components •Nameserver. •Resolver.
This performs task of looking up names. Usually, there is one nameserver for a cluster of machines. If nameserver does not contain requested information, it will contact another nameserver. But it is not required for every server to know how to contact every other server. Every nameserver will know how to contact root nameserver, and this in turn will know location of every authoritative nameserver for all second level domains.
This runs on a client machine to initiate DNS lookups. It contains a list of nameservers to use. As we have read, function of each of these nameservers is to resolve name queries. There are three types of nameservers-primary nameserver, secondary nameserver, and caching nameserver. The secondary nameservers are configured for backup purposes. Caching nameservers only resolve name queries but do not maintain any DNS database files. It is important to note here that any change to primary nameservers needs to be propagated to secondary nameservers. This is because primary nameservers own database records. The changes are propagated via a ‘zone transfer'.