IGMP, the Internet Group Management Protocol, has IP protocol number 2.
The message format for v1 (rfc1112) was
For v2 (rfc2236) this changed to:
A Host Membership Query is sent to the all hosts IP group address (220.127.116.11) with TTL 1 by a multicast router on startup and at intervals (not usually less than 60s) thereafter. All hosts in a multicast group respond to a Host Membership Query with a Host Membership Report (or send one when they want to join a multicast group for the first time). For ICMP v2 the IP TTL must be 1 and the IP Router Alert Option (rfc2113) must be set in the IP header. In v2 each multicast router on a single network will assume the role of either "Querier" or "Non-Querier"; the multicast router with the lowqest IP address should be the Querier. When a host leaves a group it must send a Leave Group message to the all-routers group (18.104.22.168) if it was the last host to reply to a Query (it may send one anyway).This page is maintained by Martin Ansdell-Smith