I usually start with a NSlookup. If that doesn't work then I jump to the whois. Often individual IP assignments will not be in the whois. Take 126.96.36.199 for an example. The whois prints up: Lycos, Inc. (NETBLK-NETBLK-LYCOS-1) 400-2 Totten Pond Road Waltham, MA 02451 US
But the NS lookup will give you: bos-fe3-4.bos.lycos.com Which could be more telling.
Often the NS lookup will be a different organization than the whois. There are a ton of NS lookup tools on the web, if you could put up with all the annoying popups you could use the tools that sits in my profile url.