Milan Technology MIL-SME801P - Manual (Page 121)

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IGMP has three fundamental types of messages:
Message Description
Query
A message sent from the querier (IGMP router or switch)
asking for a response from each host belonging to the
multicast group.
Report
A message sent by a host to the querier to indicate that the
host wants to be or is a member of a given group indicated in
the report message.
Leave Group
A message sent by a host to the querier to indicate that the
host has quit to be a member of a specific multicast group.