Installing and Administering LAN/9000 Software

138 Chapter6
Network Addressing
IP Multicast Addresses
IP Multicast Addresses
IP multicasting provides a mechanism for sending a single datagram to a
group of systems. Generally, only systems that have joined the multicast
group process the datagrams.
Multicast datagrams are transmitted and delivered with the same “best
effort” reliability as regular unicast IP datagrams. The datagrams are
not guaranteed to arrive intact at all members of the destination group
or in the same order as the datagrams were sent.
Membership in a multicast group is dynamic. Systems can join or leave
groups at any time based upon the applications’ behavior. A system
remains a member of a multicast group until the last socket that joined
the group is closed or drops membership in the group. A system can be a
member of more than one group at a time. A system that has multiple
interfaces might be a member of the same group on each interface.
IP Multicast Addresses
At the IP layer, a multicast address is a Class D Internet address with
the following format:
Figure 6-4 Multicast Address Format
User IP multicast addresses can be in the range 224.0.1.0 through
239.255.255.255. The addresses 224.0.0.0 through 224.0.0.255 are
reserved. The addresses of other well-known permanent multicast
groups are published in the “Assigned Numbers” RFC (RFC-1060, March
1990).
Ethernet Multicast Addresses
The Ethernet data-link address, also called the link level or station
address, is derived from the IP multicast address. The lower 23 bits of
the IP multicast address are placed into the lower 23 bits of the Ethernet
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MULTICAST GROUP ADDRESS