because it simple...

reverse dns lookup,reverse ip/domain lookup

Main content page - reverse dns lookup.Network administrators often refer to a reverse lookup as reverse resolving, or more specifically reverse DNS lookup, and is accomplished using a "reverse IN-ADDR entry" in the form of a PTR record.
This memo describes the service mechanism for entering a delegation of DNS servers that provide reverse lookup of 6to4 IPv6 addresses into the 6to4 reverse zone file. The mechanism is based on a conventional DNS delegation service interface, allowing the service client to enter the details of a number of DNS servers for the delegated domain.
From there I do a reverse DNS (rDNS) lookup to see what hostname the ISP provides. A hostname is something like www.example.com, or can be more specific to include regional information.
Reverse DNS turns an IP address into a hostname -- for example, it might turn 192.0.2.25 into host.example.com. Reverse DNS (rDNS) is a method of resolving an IP address into a domain name, just as the domain name system (DNS) resolves domain names into associated IP addresses.
Reverse DNS lookups for IPv4 addresses use a reverse IN-ADDR entry in the special domain in-addr.arpa. In the in-addr.arpa domain a sequence of bytes in reverse order represent an IPv4 address, encoded as decimal numbers, separated by dots with the suffix .