With IP multicast, router may receive one packet and forward several copies. For IPv4, Class D addresses are reserved for multicasting. The 32-bit Class D addresses begin with 1110 binary, so they range from 224.xx.yy.zz to 239.xx.yy.zz. IGMP report message sent by host on a LAN to join a multicast group. Within the autonomous system, each router learns about all multicast groups from OSPF routing packets which are flooded to all routers in the autonomous system.
Sparse mode follows.
Router becomes a destination router for a given multicast group when when at least one local host joins the group using IGMP or a similar 4. Router becomes a source router when one or more local hosts are transmitting on the multicast group address via the router. Group destination routers send join messages toward the RP using unicast shortest path routing. The reverse path becomes the initial distibution tree to listeners. Initially, packets travel from the source router to the rendezvous point, which serves as the root of the distribution tree to the destination routers.