Operation Manual – MulticastH3C S7500 Series Ethernet Switches Chapter 1 Multicast Overview1-8Note:Like having reserved the private network segment 10.0.0.0/8 for unicast, IANA has alsoreserved the network segments ranging from 239.0.0.0 to 239.255.255.255 formulticast. These are administratively scoped addresses. With the administrativelyscoped addresses, you can define the range of multicast domains flexibly to isolate IPaddresses between different multicast domains, so that the same multicast addresscan be used in different multicast domains without causing collisions.II. Ethernet multicast MAC addressWhen a unicast IP packet is transmitted in an Ethernet network, the destination MACaddress is the MAC address of the receiver. When a multicast packet is transmitted inan Ethernet network, a multicast MAC address is used as the destination addressbecause the destination is a group with an uncertain number of members.As stipulated by IANA, the high-order 24 bits of a multicast MAC address are 0x01005e,while the low-order 23 bits of a MAC address are the low-order 23 bits of the multicastIP address. Figure 1-4 describes the mapping relationship:XXXX XXXXX XXXX XXXX XXXX1110 XXXX0XXX XXXX XXXX XXXXXXX XXXXX XXXX XXXX0000 0001 0000 0000 0101 111032-bit IP address48-bit MAC address5 bits lost25-bit MAC address prefix… …23 bitsmappedFigure 1-4 Mapping relationship between multicast IP address and multicast MACaddressThe high-order four bits of the IP multicast address are 1110, representing the multicastID. Only 23 bits of the low-order 28 bits are mapped to a MAC address Thus five bits ofthe multicast IP address are lost. As a result, 32 IP multicast addresses are mapped tothe same MAC address.1.2.2 IP Multicast ProtocolsIP multicast protocols include multicast group management protocols and multicastrouting protocols. Figure 1-5 describes the positions of multicast-relevant protocols inthe network.
