Enter the IP address here:
http://www.arin.net/whois/
It will tell you what prganization owns it. Usually this is an Internet
provider or a large corporation/institution. You won't really be able to
see who is actually *using it* (you would have to contact the owner of the
address block if you really need that information, and I doubt they will
provide it to you without a valid reason such as a court order.)