Reverse DNS lookup (rDNS)

Have you ever wanted to know which URL belongs to an ip address? For example, what is the URL of the Google public DNS service, 8.8.8.8?

To overcome to this problem I wrote a program to do the reverse DNS lookup (rDNS), of a given IP address, here it is:

For example, looking 8.8.8.8, you get google-public-dns-a.google.com