This paper describes the IMcast (Implicit Multicast) routing protocol, developed to address several problems that are responsible for the slow deployment of IP Multicast. We focus on transparent integration with legacy routers and in improving the efficiency of the routing tree. IMcast packets carry unicast addresses in the destination field of IPv4 header and multicast addresses on a separate header. As a result, one can integrate IMcast routers into the Internet without changes to existing routers. The address space for multicast connections is separate and independent of unicast addresses. IMcast builds and maintains shortest path trees from the sender to the receivers. The performance of its control plane scales well. IMcast can also be used as an automatic tunneling protocol to connect existing islands of IP multicast in the Internet. The routing tree construction algorithm has been validated with an implementation.