### IP Address Lists and Station Number Portability

The 9600 Series IP Telephones provide the capability to specify IP address lists. On startup or a reboot, the telephone attempts to establish communication with these various network elements in turn. The telephone starts with the first address on the respective list. If the communication is denied or times out, the telephone proceeds to the next address on the appropriate list and tries that one. The telephone does not report failure unless all the addresses on a given list fail, thereby improving the reliability of IP telephony.

This capability also has the advantage of making station number portability easier. Assume a situation where the company has multiple locations in London and New York, all sharing a corporate IP network. Users want to take their telephones from their offices in London and bring them to New York. When users start up their telephones in the new location, the local DHCP server usually routes them to the local call server. With proper administration of the local DHCP server, the telephone knows to try a second call server IP address, this one in London. The user can then be automatically registered with the London call server.

Chapter 5: Server Administration contains details on administration of DHCP servers for lists of alternate media servers, router/gateways, and HTTP/HTTPS servers. For more information, see DNS Addressing.