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 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.




	NOTE: Several IP multicast addresses may share the same Ethernet multicast address because the IP multicast address has 28 significant bits.

Multicast Routing

Multicast datagrams are sent through the interface associated with the default route. If that interface does not support multicast, attempts to send multicast datagrams will result in the ENETUNREACH error.

A default multicast route can be configured by specifying a network route for 224.0.0.0. The example below provides both the command line and netconf file entries.

route add 224.0.0.0 192.1.2.3 0 #192.1.2.3 is a local interface

ROUTE_DESTINATION[1]="224.0.0.0"
ROUTE_GATEWAY[1]="192.1.2.3"
ROUTE_COUNT[1]="0"

Additionally, routes for specific multicast addresses can be configured just like any other host route. The example below provides both the command line and netconf file entries.

route add 224.1.2.3 192.5.6.7 0 #192.5.6.7 is a local interface

ROUTE_DESTINATION[2]="224.1.2.3"
ROUTE_GATEWAY[2]="192.5.6.7"
ROUTE_COUNT[2]="0"

IP Multicast Addresses

Technical documentation

» Table of Contents