Multicast enables the efficient use of network bandwidth by sending the minimum number of datagrams to the maximum number of nodes. On a local link, one multicast datagram is sent to a possibly unlimited number of nodes. A special address prefix3 identifies a multicast datagram, and a specific address inside that prefix identifies each group of nodes. For example, a videoconference would have one specific multicast destination address that each node, listening to the videoconference, processes these multicast datagrams. This multicast address is often called a multicast group. Multicast is a one-to-many addressing mechanism.
Broadcast is a mechanism where all nodes should listen to every broadcast datagram sent to the broadcast address.4 Even nodes that do not care for this specific datagram are interrupted in order to process the broadcasted datagram. For example, routers exchange routing information through broadcasts that cause an interrupt on all hosts, including those that do not care about route exchanges.
IPv4 uses broadcasts for many purposes but IPv6 does not, using instead multicast. This enables more fine grain targets of datagrams that have to be sent to many nodes on the network. For example, IPv6 routers exchange information using a specific multicast address identifying all routers. A host will not listen to that specific multicast address since it is not a router.
The scope of multicast groups is important in order to limit the forwarding of ...