1. A method of enabling communication on an Internet Protocol based network which comprises: establishing a communications connection between a communications device and a server via a network; querying a database associated with the server, the database containing data relating to free Session Initiation Protocol ID's that are available to be allocated to the communications device and data relating to allocated Session Initiation Protocol ID's, for determining a free Session Initiation Protocol ID that can be allocated to the communications device; allocating the determined free Session Initiation Protocol ID for use by the communications device; moving the determined free Session Initiation Protocol ID to the data relating to allocated Session Initiation Protocol ID's; and after the communications device to which the Session Initiation Protocol ID was originally allocated has completed a communications session using SIP, returning the Session Initiation Protocol ID to the data relating to free Session Initiation Protocol ID's for use by another communications device.