When a router gets a multicast datagram from another network and none of its subnets have any members of the group address, it discards the packet.
A host joins a multicast group for a multicast address when it is set up to accept datagrams transmitted to that address.
A group may contain one or as many hosts as desired. The names of specific group members are not kept on file by hosts or routers.
A host has the ability to send to many multicast addresses and participate in different multicast groups.
A host does not have to be a member of a group in order to transmit multicast datagrams to that group's address, and a host can send datagrams to a multicast group address even if that group has no members.
Using IGMP, a router will check to see whether any hosts on a locally linked network are set up to accept multicast datagrams ( Internet Group Management Protocol).
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