3Com Switch 8800 Configuration Guide Chapter 23 IP Multicast Overview23-6Class D address range Description…… ……II. Ethernet Multicast MAC AddressesWhen a unicast IP packet is transmitted on the Ethernet, the destination MAC addressis the MAC address of the receiver. However, for a multicast packet, the destination isno longer a specific receiver but a group with unspecific members. Therefore, themulticast MAC address should be used.As Internet Assigned Number Authority (IANA) provisions, the high 24 bits of amulticast MAC address are 0x01005e and the low 23 bits of a MAC address are the low23 bits of a multicast IP address. The high twenty-fifth bit is 0, a fixed value.1110XXXX XXXXXXXX XXXXXXXX XXXXXXXXXXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXXXXXXXXXX XXXXXXXX32-bit IPaddress5 bitsunmapped23 bits mapped48-bit MACaddressFigure 23-4 Mapping between a multicast IP address and an Ethernet MAC addressThe 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 fivebits are lost. This may results in that 32 IP addresses are mapped to the same MACaddress.23.2.2 IP Multicast ProtocolsIP multicast protocols mainly involves multicast group management protocols andmulticast routing protocols. Their application positions are shown in Figure 23-5Application positions of multicast-related protocols.
