IGMP
When a host wishes to join a multicast group, it sends an Internet Group
Management Protocol (IGMP) message to the router. The router periodically
checks for group members on each segment. There are three versions of IGMP.
IGMP Version 1
Multicast routers query each segment periodically to see if there are still
hosts in multicast groups with a query sent to the all-hosts address of
224.0.0.1. One host on the segment responds. Hosts silently leave a group;
the router doesn�ft know they are gone until it queries and no one responds.
IGMP Version 2
Version 2 adds explicit leave messages that hosts send when they leave a
group. Queries are sent to specific multicast group addresses, not the allhosts
address.
IGMP Version 3
Hosts are able to tell the router not only which multicast groups they belong
to, but also from which sources they will accept multicasts. It adds two
modes for requesting membership in a multicast group:
�¡ Include mode.The receiver lists the group or groups to which it will
belong and the servers it will use.
�¡ Exclude mode.The receiver lists the group or groups to which it will
belong and the servers it will not use.