HP-UX LAN Administrator's Guide (Feburary 2007)

DRAFT COPY Network Addressing
IP Multicast Addresses
Chapter 7 133
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 7-15 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 multicast address 01-00-5E-00-00-00 (hex). Ethernet multicast
addresses can be in the range 01-00-5E-00-00-01 through 01-00-5E-7F-FF-FF.
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MULTICAST GROUP ADDRESS