How membership messages are used with IGMP

Multicast routers use IGMP for enquiring periodically which multicast groups have members in the connected local networks.

This inquiry is carried out by sending Host Membership Query messages to the all-hosts address 224.0.0.1. The hosts receiving the query respond by sending Host Membership Reports to all neighboring multicast routers.

A host joining a new group immediately transmits a report, instead of waiting for a query. When a host wants to stop receiving a multicast transmission, it sends a Leave Report message with the destination address 224.0.0.2 to all subnet routers. A router receiving a Leave Report message sends in response a Group Specific Query to the multicast address to check whether there still are hosts in that group. In case no response is received, multicasting to that address is stopped.