The FREE REVERSE DNS LOOKUP resolves an IP address to a hostname.
Example :

• Reverse DNS resolves  192.0.2.25 to host.example.com (an IP address into a host name).
• Classic reverse DNS lookup path: DNS resolver => root servers => ARIN (North American IP registry) => Local ISP => Acme Inc. DNS servers.
• Whoever provides your IP addresses (usually your ISP) MUST either
• set up your reverse DNS entries on their DNS servers
• "delegate authority" for your reverse DNS entries to your DNS servers.
• Reverse DNS entries use a host name with a reversed IP address with ".in-addr.arpa" added to it -- for example, "25.2.0.192.in-addr.arpa" (".ip6.arpa" is used for IPv6 reverse DNS lookups).
• Reverse DNS entries are set up with PTR records (whereas standard DNS uses A records), which look like "25.2.0.192.in-addr.arpa. PTR host.example.com" (while standard DNS would look like "host.example.com. A 192.0.2.25").
• All Internet hosts should have a reverse DNS entry (see RFC1912 section 2.1).
  

Enter Ip address eg. 192.168.1.1