In the cyberworld every domain name is associated with a unique IP address. While the domain name is human recognizable (alphanumeric), the IP address is machine-readable. An IP address is a set of four numbers (octets) separated by dots.  The Internet’s domain name system (DNS) maps the domain names to corresponding IP addresses to serve web pages to Internet surfers.

Within a particular top-level domain there are millions of domain names and each of these corresponds to a unique IP address. Similarly, there are many different top-level domains. So the domain name system maintains the details of all these different domains and their IP addresses on a distributed database. This is implemented through a number of name servers located across the world. These name servers can either be dedicated devices or software running on machines that perform a variety of other tasks. The name server(s) for a particular domain or TLD stores and manages all the DNS records for that domain including any subdomains.

One of the main functions of a name server is responding to resolution requests. When a web surfer types in a URL into a browser address bar, the browser will need to know its corresponding IP address to request the particular web page from the machine at that IP address. For this, the browser approaches the default name server of the web surfer’s machine with a resolution request. The name server then translates the URL into an IP address. This may be done in steps by contacting one of the root name servers first to get the IP address of the name server of the requested TLD and then asking this name server for the IP address of the machine hosting the particular domain. Finally, the browser has the IP address to request the web page from the machine at that address. This kind of mapping from a domain name to an IP address is the most common and is termed as forward DNS lookup.

The exact opposite of this process is known as reverse DNS lookup. This lookup returns the domain name for a particular IP address. Reverse DNS lookup is often employed in tracing out spammers and hackers.