This paper specifies a multicast routing protocol called Distributed Core Multicast (DCM). It is intended for use within a large single Internet domain network with a very large number of small multicast groups. We present how DCM can be used to route packets to mobile hosts in cellular Internet telephony, where each mobile host is assigned a multicast address. The benefits of the multicasting-based approach to route packets to mobile hosts are low latency and no disruption during handover.