Internet Group Management Protocol (IGMP) manages the membership of hosts and routers in multicast groups. IP hosts use IGMP to report their multicast group memberships to any immediately neighboring multicast routers. Multicast routers use IGMP to learn, for each of their attached physical networks, which groups have members.
IGMP is also used as the transport for several related multicast protocols (for example, DVMRP and PIMv1), and the mtrace utility.
IGMP is an integral part of IP and must be enabled on all routers and hosts that want to receive IP multicasts.
For each attached network, a multicast router can be either a querier or a nonquerier. The querier router periodically sends general query messages to solicit group membership information. Hosts on the network that are members of a multicast group send report messages. When a host leaves a group, it sends a leave group message.