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IGMP allows a host to register group memberships with the local querier router to receive any datagrams sent to this router and targeted to a group with a specific IP multicast address. IGMP also allows a router to learn the existence of group members on networks to which it is directly attached. The router periodically sends a general query message to each of its local networks. Any host that is a member of any multicasting group identifies itself by issuing IGMP reports.