You can look up an IP address on WhoIs by clicking on the Reverse IP Address Lookup tab located under the list of tools. Using this reverse lookup allows you to view any domain hosted at that location.
You can either enter a complete or partial IP address. Using a "%" at the end of a partial address broadens the search. There are limitations to this tool, however. For example, recently removed domains may still appear under a particular IP address for two weeks after their removal. This tool also allows you to enter a domain name and view its IP address along with any other domains that share the same address.