The process of a resolver querying a name server to resolve a host’s IP address into its associated fully qualified domain name (FQDN). This is the reverse of the usual host name resolution process, in which a resolver queries a name server to resolve a host name into its associated IP address. Reverse name lookups use a special domain called in-addr.arpa.
Reverse name lookups are used in a variety of circumstances. For example, when a Web browser contacts a Web server, the Web server obtains the IP address of the computer the browser is running on. The Web server software often uses a reverse lookup to try to resolve the client’s IP address into its associated FQDN, usually for purposes of logging the Hypertext Transfer Protocol (HTTP) session.