whois is a program that will tell you the owner of any second-level domain name who has registered it with Verisign (or with Network Solutions, which was acquired by Verisign). Network Solutions was originally the only Internet registrar of the com, net, and org domain names) and many domain names are still registered with Verisign. If a Web site obtained its domain name from Network Solutions or Verisign, you can look up the name of the owner of the Web site by entering (for example):
and whois will tell you the owner of that second-level domain name.
whois can also be used to find out whether a domain name is available or has already been taken. If you enter a domain name you are considering and the search result is "No match," the domain name is likely to be available and you can apply to register it.
Recently, the Internet Corporation for Assigned Names and Numbers (ICANN) has opened up domain name registration to a number of other companies. To search all of these companies at the same time for registration information, you can use BetterWhois.