3Com 8807 Configuration Manual page 413

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IP Multicast Protocols

Table 408 Reserved multicast address list

Class D address range Description

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

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 mainly involves multicast group management protocols and

multicast routing protocols. Their application positions are shown in Figure 100

"Application positions of multicast-related protocols".

All PIM routers

RSVP encapsulation

All CBT routers

Specified SBM

23 bits mapped

Implementation of IP Multicast

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