3com 8807 User Guide

Implementation of IP Multicast

413

Ethernet Multicast MAC Addresses

When a unicast IP packet is transmitted on the Ethernet, the destination MAC
address is the MAC address of the receiver. However, for a multicast packet, the
destination is no longer a specific receiver but a group with unspecific members.
Therefore, the multicast MAC address should be used.

As Internet Assigned Number Authority (IANA) provisions, the high 24 bits of a
multicast MAC address are 0x01005e and the low 23 bits of a MAC address are
the low 23 bits of a multicast IP address. The high twenty-fifth bit is 0, a fixed
value.

Figure 99 Mapping between a multicast IP address and an Ethernet MAC address

The first four bits of the multicast address are 1110, representing the multicast
identifier. Among the rest 28 bits, only 23 bits are mapped to the MAC address,
and the other five bits are lost. This may results in that 32 IP addresses are mapped
to the same MAC address.

IP Multicast Protocols

IP multicast protocols mainly involves multicast group management protocols and
multicast routing protocols. Their application positions are shown in Figure 100
“Application positions of multicast-related protocols”.

224.0.0.13

All PIM routers

224.0.0.14

RSVP encapsulation

224.0.0.15

All CBT routers

224.0.0.16

Specified SBM

224.0.0.17

All SBMS

224.0.0.18

VRRP

......

Table 408 Reserved multicast address list

Class D address range Description

1110XXXX

XXXXXXXX

32-bit IP

address

5 bits

unmapped

23 bits mapped

48-bit MAC

address