A computer system that hosts a cable modem is described. The cable modem may be used to send and receive messages over the Internet using a cable network managed by a cable operator. Unlike external modems, the cable modem uses the same CPU and memory that are used by the computer system's operating system. However, the cable modem has a different network address than the computer system. When the cable modem receives a data packet having the network address of the cable modem, a cable modem driver forwards the packet to a protocol stack for the cable modem. When the cable modem receives a data packet having the network address of the computer system, the packet is forwarded to a different protocol stack for the computer system. Since the protocol stacks of the cable modem and computer system are different, direct communication from the computer system to the cable modem is not permitted.