Messages ABOUT groups are sent on the special all hosts group 184.108.40.206
Time to live: limits the distance messages travel.
Flood & Prune: All routers get packets initially, then prune out parts of the network that donít have group members.
Tunneling: encapsulates multicast packets in regular packets in order to pass through routers that do not support IP Multicast.
Extensions to IP between the routers:
Distance Vector Multicast Routing Protocol (DVMRP, PIM-DM)