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Internet Group Multicast Protocol (IGMP) is used to manage one-to-many Internet Protocol sessions utilizing the multicast protocol and multcast address range (224.0.0.0 through 239.255.255.255). Like ICMP, IGMP is implemented as part of the IP software, but runs as a "higher level" protocol. IGMP datagrams are encapsulated within the IP datagram's payload. IGMP is now in it's third revision as a proposed standard (not yet ratified, yet it is in use).

Here is how the networks stack looks when encapsulated

• Message traffic
• Internet Group Management Protocol (encapsulates multicast application message)
• Internet Protocol (encapsulates IGMP)
• Network hardware (Physical and Datalink layers - Usually Ethernet/LLC, segments and encapsulates IP)

Routers use IGMP to manage multicasting. The IGMP messages are used to learn which hosts are part of which multicast groups. Hosts use IGMP to subscribe to and leave from multicast groups.

• IGMP Messages
• IGMP Message Format
• IGMP Operation
• IGMP Snooping

IGMP STANDARDS

• RFC 1112 - Host Extensions for IP Multicasting, Appendix I
• RFC 2236 - Internet Group Management Protocol, v2