Whoisis a TCP-based query/response protocol which is widely used for querying an official database in order to determine the owner of a domain name, an IP address, or an autonomous system number on the Internet. WHOIS lookups were traditionally made using a command line interface, but a number of simplified web-based tools now exist for looking up domain ownership details from different databases.

The WHOIS system originated as a method that system administrators could use to look up information to contact other IP address or domain name administrators (almost like a "white pages"). The use of the data that is returned from query responses has evolved from those origins into a variety of uses including: