enumeration using “whois” ….

whois is a TCP service, a tool and a database. Whois databases contain information about nameservers, registrars, and contact information. It can be used to perform different kinds of forward and reverse lookups; both of which will be explained in this article.Read more

Consider the command
whois checkpoint.com

Please note that there is NO malicious intent in the choice of site in the above example.

This is a typical example of a forward whois lookup. In this manner, you can get essential details like information on the registrar, the whois server, the nameservers, the domain name and contact information like telephone or fax numbers.

Now consider
whois 216.200.241.66

the command does a reverse lookup on the ip “216.200.241.66″. Doing such lookups can help in returning the ip address range of a particular domain.

here are also websites which provide the whois lookup service. some are :-
www.whois.com
www.completewhois.com

whois lookups can be performed on most linux based systems using the terminal but i personally suggest that you use backtrack for the very same;never heard of backtrack?