IGMP
IGMP: “signaling” protocol to establish, maintain, remove groups on a subnet.
Router sends Host Membership Query to 224.0.0.1 (all multicast hosts on subnet)
Host responds with Host Membership report for each group to which it belongs, sent to group address
Membership report => other hosts in the same group “suppress” reports
Router periodically broadcasts query to detect if groups have gone away
Asynchronous reports possible.