Sometimes you want to identify a SPAM message source or just to find out who left
that blog comment you might want to know to whom the IP block has been assigned. Though
it is difficult to pinpoint it to an individual organization (especially SMEs who
will have only few IP numbers or Dial-up/Home users where the IPs will be in their
ISP's names) there is a service that can give you some information. You can American
Registry for Internet Numbers Whois
Service or Asia-Pacific NIC's Whois
Service. Use one of the services and input the IP address for which you are interested
to find the block owner.
Read the complete post at http://www.venkatarangan.com/blog/2007/07/25/Who+Does+The+IP+Belong+To.aspx