To receive a particular multicast data stream, hosts join a multicast group. The group is identified by its multicast address. The communication between the host and router for this purpose is managed by using the Internet Group Management Protocol (IGMP).
Hosts send an IGMP Join message to their local multicast router, signaling their intention to join the group (G), as represented by a multicast address. The source device (S) that delivers the content sends the message addressed to the multicast address of G. The multicast “channel” is the combination of the IP address of the content source and multicast address of the group—called an (S, G) pair. For messages for which the source can be any device, the S is replaced by the asterisk wildcard (“*”), which means “any source.”