One of the of SIP server functions is users location and names resolution. Normally, the user agent does not know the IP address of the called person, it only knows its "e-mail".

SIP entities identify a user by its SIP URI (Uniform Resource Identifiers) defined in RFC 2396. A SIP URI has a format similar to the one of the email, consists of a user and a domain delimited by one @, as it shows in the following examples:


user@domain, where domain is a full domain name.
user@machine, where machine is the name of the machine.
user@ip_address, where address is the IP address of the machine.
telephone_number@gateway, where the gateway allow to access through the PSTN to the called number

The SIP identification solution, can be also based on the DNS described in RFC 3263, where it is described the DNS procedures used by the clients to translate a SIP URI in an IP address, port and transport protocol or used by the servers to return an answer to the client in case that the request fails.