A TCP/IP network layer protocol used for informing routers of the availability of multicast groups on the network.
In a multicasting environment, Internet Group Management Protocol (IGMP) is used to exchange information on the status of membership in multicast groups between routers on the network. In other words, once a router becomes aware that there are hosts on a locally attached network that are members of a particular multicast group, it advertises this information to other routers on the internetwork so that multicast messages are forwarded to the appropriate routers.
To join a multicast group, a host must report its request for membership to nearby routers. These routers periodically poll the hosts in their locally attached networks to check on their membership status. When a host first joins a multicast group, it sends an IGMP message called a Host Membership Report that is directed to the multicast address 244.0.0.1 containing the multicast address that identifies the group it wants to join. Routers connected to that host’s local network then advertise to other routers throughout the internetwork that the particular network has hosts belonging to that multicast group.
The routers poll the hosts regularly by sending Host Membership Query messages to determine whether any of them are still members of that group. If no hosts on the network belong to that group any longer, the router stops advertising the information to other routers on the internetwork so that multicast messages directed to that group are no longer forwarded to it.
IGMP is used in Microsoft Windows NT and Windows 2000 only when the Windows Internet Naming Service (WINS) is installed. At startup, a WINS server sends IGMP packets to the multicast address 220.127.116.11 to seek out possible WINS replication partners on the network.