Reverse IP lookup is a technique that is used to probe any given IP address for all the domains it hosts. So all you need to do is feed the target IP address and then you'll be returned to all the domains hosted on that IP address. One such tool for reverse IP lookup is available online at http://www.yougetsignal.com/tools/web-sites-on-web-server/.
Reverse IP lookup works only on Internet-facing websites and isn't applicable for sites hosted on intranet.