If you have an IP Address, and you wish to know the Domain Name associated with it, you can perform the reverse of the usual DNS process (where you know the name and need the IP address - this is what web browsers do when you type in WWW.EVERYTHING2.COM, for example).

There are many tools to perform a Reverse DNS lookup, but one handy web-based one is at http://www.osilab.ch/services/dns_e.htm

Other handy search services can be found at the very useful http://www.searchgateway.com/finde.htm, or Windows users should get the highly recommended NeoTrace program.