Multicasting
Multicast has been available in IPv4 since 1988 and is used to address a certain group of hosts at the same time. Instead of sending out a broadcast, which is not routable and has to be processed by every node on the subnet, the multicast packet is addressed to a multicast group address out of the Class D address range. Only the hosts that are members of that multicast group will process the packet. Internet Group Management Protocol (IGMP) manages group memberships and routing of IPv4 multicast packets. The deployment of IPv4 multicast capabilities was supported by the MBONE project, started in 1992. The MBONE is a virtual network layered on top of portions of the physical Internet to support routing of IP multicast packets; it was created because that function had not yet been integrated into many production routers. The network is composed of islands that can directly support IP multicast, such as multicast LANs (like Ethernet) linked by virtual point-to-point links (tunnels).
In IPv6, multicast is an integral part of the protocol and available
on all IPv6 nodes. A new multicast address format has been defined
with added functionality by using scopes in addition to the group
address. For example, a multicast group address can be in a
link-local scope (FF02), in a site-local scope
(FF05), or in a global scope
(FF0E). For an explanation of the multicast
address format and a list of scope identifiers, refer to Chapter 3.
The functionality of IGMP has been incorporated ...
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