In network multimedia applications, such as multiparty teleconferencing, users often need to send the same information to several other users. To manage such one-to- many or many-to-many communication efficiently in wide-area internetworks, it is imperative to support and perform multicast routing. Multicast routing sends a single copy of a message from a source to multiple receivers over a communication link that is shared by the paths to the receivers. Loop-freedom is a specially important consideration in multicasting. Because applications using multicasting tend to be multimedia and bandwidth intensive, and loops in multicast routing duplicates looping packets. We present a new multicast routing protocol, called multicast internet protocol (MIP), which offers a simple and flexible approach to constructing both group-shared and shortest-paths multicast trees. MIP can be sender-initiated or receiver-initiated or both; therefore, it can be tailored to the particular nature of the an application's group dynamics and size. MIP is independent of the underlying unicast routing algorithms used. MIP is robust and adapts under dynamic network conditions to maintain loop-free multicast routing. Under stable network conditions, MIP has no maintenance or control message overhead.