IGMP on the Gateway Router
Internet Group Management Protocol, or IGMP, is the protocol used to manage group subscription for IPv4 multicast. On the gateway router, called the querier, IGMP is used to track multicast group subscriptions on each segment. The router sends query messages to discover the hosts that are members of a group. The hosts send membership report messages to inform the router that they are interested in receiving or leaving a particular multicast stream, and they also send report messages in response to a router query message.