A method of authenticating a user in an IP Multimedia Subsystem network, the method comprising receiving from an access network an access identifier defining a terminal's physical location, retrieving from a database a registered access identifier associated with the user; and determining if the received access identifier matches the registered access identifier, and if so then authenticating the user in the IMS network, and if not then performing an alternative authentication method. The method allows a nomadic user in the access network to register with an IP Multimedia Subsystem network.