Automatic compilation of address filter information permits a cable modem to route incoming data packets to destination devices. A cable modem uses address filter information, such as for example, a list of addresses of communication devices that have previously registered with the cable modem, to identify incoming data packets that are addressed to communication devices associated with the cable modem. Incoming data packets with addresses that are not in the address filter information are filtered out. However, the cable modem may have insufficient filter information, when, for instance, a new communication device is associated with the cable modem. In order to automatically register communication devices associated with the cable modem, the cable modem inspects the source addresses of outgoing data packets. If the source address is not included in the address filter information, the source address is added.