Multicast Group
A multicast group uses an IP multicast address identifier. Any host (or other receiving device) that joins a multicast group becomes a member of the group and can identify and receive an IP packet whose destination address is the IP multicast address.
A source that has a multicast destination address for forwarding IP packets is called a multicast source.
A multicast source can simultaneously send data to multiple multicast groups.
Multiple multicast sources can simultaneously send data to a multicast group.
The members of a multicast group are dynamic. Hosts can randomly join and leave a multicast group in the network. Members may be widely distributed in any place of the network.
The multicast source is generally not the receiver or a member of a group.
The following example compares multicast transmission to a TV channel:
The multicast group is the link between a sender and a receiver, like a TV channel.
The TV station acts as the multicast source and sends packets to the channel.
The TV is the receiver host. Audiences turn on the TV and select the program of their choice on a channel, which is like hosts joining a group. The TV plays the channel's programs, which is like hosts receiving data sent to the group.
Audiences can turn the TV on or off or switch channels randomly, just as hosts dynamically join or leave a multicast group.
Based on the distribution of members, the multicast routing protocol sets up routing in the shape of a tree for forwarding data packets with multiple destinations. Packets are copied and distributed at the farthest branch from the source and are sent to members of the multicast group.
