There are three message types used in IGMP. The IGMP 'type' field is set to the following values for each message type [ in hex ] :
MEMBERSHIP QUERY [ 0x11 ]
Membership Query messages are used by multicast enabled routers running IGMP to discover which hosts on attached networks are members of which multicast groups. Membership Query messages are sent to the 'all-systems' multicast group address of 224.0.0.1.
There are two types of Membership Queries:
General Query - used to learn which groups have members on an attached network.
Group-Specific Query - used to learn if a specific group has any members
on an attached network.
MEMBERSHIP REPORT (v1/v2) [ 0x12 / 0x16 ]
A membership report message is sent by a host whenever it joins a multicast group, and when responding to Membership Queries sent by an IGMP router that is functioning as a Querier.
LEAVE GROUP [ 0x17 ]
This message is sent when a host leaves a multicast group. This message is sent to the 'all-routers' multicast address of 224.0.0.2. The router then sends out a group-specific membership query to the network to verify if the last member of a group has left.