Reverse DNS Lookup, Test reverse DNS (PTR) by IP address or hostname

What's Reverse DNS?
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 .in-addr.arpa. For example, the reverse lookup domain name corresponding to the IPv4 address 10.12.13.140 is 140.13.12.10.in-addr.arpa. A host name for 1.2.3.4 can be obtained by issuing a DNS query for the PTR record for that special address 4.3.2.1.in-addr.arpa. Read more on wiki