Enter the full IP (and the code that stops scripts from getting through and using the site automatically) and it will return the name of the owner of the IP range in question. If its someone's home account it will usually be an ISP, and if its a company it may list the name of the relevant company. Sometimes, though, it gives an ISP's name even though it belongs to a company - since the the ISP may have registered the IP on behalf of that company.
Clear as mud? There are other tools on the site if you go back a level on the URL I sent you.