Multicast tunnels are used to connect multicast routers together across the standard Internet. The router at the other end of the tunnel then unencapsulates it and sends it on its way. This allows portions of the network that support multicast to interconnect themselves across portions of the network that do not support multicast. Another feature of multicast is its ability to take advantage of broadcast mediums. Broadcast mediums are communications systems where each system connected to the medium can receive each message sent in the channel.
Ethernet is an example of a broadcast medium.
Each packet of information sent across an Ethernet is heard by every computer connected to the Ethernet. Typically Ethernet cards monitor the channel and only interrupt their host processor with packets addressed to the host computer. Ethernet cards can also be configured to watch for packets addressed to a particular multicast address. Then each time it sees a packet with that multicast address it delivers it to its host computer. This allows a multicast router on a local area network to communicate identical information to a set of computers in a multicast group without duplicating information on the network.