What is Reverse IP, Domain Lookup?

Reversing an IP to DNS is a way to look at how many websites share the same IP address. As for example, every active domain name resolves to an IP address. Every IP address, in turn, resolves to a server name. Thus by taking a look at the IP address a domain uses, you can figure out the other domains that rely upon it and, thus, get a good picture of the other domains on the server.

It may help in filtering the websites created by spammers. Since most web hosts will let you use multiple domains with the same account, spammers simply fill up their accounts with hundreds, even thousands of domains and subdomains in a bid to create as many separate sites as possible, all of it automatically. So most of the time you can figure out if different websites (excluding the subdomains and related stuff) share the same IP.