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3Com Switch 8800 Configuration Guide Chapter 23 IP Multicast Overview
23-6
Class D address range Description
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II. 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.
1110XXXX XXXXXXXX XXXXXXXX XXXXXXXX
XXXXXX XX XXXXXXXX XXXXXXXX XXXXXXXXXXXXXXXX XXXXXX XX
32-bit IP
address
5 bits
unmapped
23 bits mapped
48-bit MAC
address
Figure 23-4 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.
23.2.2 IP Multicast Protocols
IP multicast protocols mainly involves multicast group management protocols and
multicast routing protocols. Their application positions are shown in
Figure 23-5
Application positions of multicast-related protocols.