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 ( 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 ( if it was the last host to reply to a Query (it may send one anyway).

This page is maintained by Martin Ansdell-Smith
using GNU vi and RCS.
Last modified $Date: 2006/07/12 12:13:32 $.