WHOIS (pronounced as the phrase who is) is a query/response protocol that is widely used for querying databases in order to determine the registrant or assignee of Internet resources, such as a domain name, an IP address block, or an autonomous system number[citation needed]. WHOIS lookups were traditionally performed with a command line interface application, but now many alternative web-based tools exist. WHOIS services are typically communicated using the Transmission Control Protocol (TCP). Servers listen to requests on the well-known port number 43.