Reverse DNS Resolution
When working on DNS, reverse dns resolution is just as important as forward dns resolution. Forward being from domain => ip and reverse being from ip => domain. A little ecerpt from Wikipedia says it all.
In computer networking, reverse DNS lookup or reverse DNS resolution (rDNS) is the determination of a domain name associated with an IP address via querying DNS – the reverse of the usual "forward" DNS lookup of an IP from a domain name.
The process of reverse resolving an IP address uses PTR records. The reverse DNS database of the Internet is rooted in the arpa top-level domain.
Although the informational RFC 1912 (Section 2.1) specifies that "Every Internet-reachable host should have a name" and that "For every IP address, there should be a matching PTR record...", it is not an Internet Standard requirement, and not all IP addresses have a reverse entry.Wikipedia