3com 4210 PWR 9-Port 3CR17341-91-ME 用户手册

C

HAPTER

 15: M

ULTICAST

 O

VERVIEW

When a unicast IP packet is transported in an Ethernet network, the destination 
MAC address is the MAC address of the receiver. When a multicast packet is 
transported in an Ethernet network, a multicast MAC address is used as the 
destination address because 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 multicast IP address. Figure 58 describes the mapping relationship:

Figure 58   Multicast address mapping

The high-order four bits of the IP multicast address are 1110, representing the 
multicast ID. Only 23 bits of the remaining 28 bits are mapped to a MAC address. 
Thus, five bits of the multicast IP address are lost. As a result, 32 IP multicast 
addresses are mapped to the same MAC address. 

This section provides only general descriptions about applications and functions of 
the Layer 2 and Layer 3 multicast protocols in a network. For details about these 
protocols, refer to the related chapters of this manual. 

Layer 2 multicast protocols include IGMP Snooping and multicast VLAN. Figure 59 
shows where these protocols are in the network. 

n

XXXX X

XXXX XXXX

XXXX XXXX

XXXX XXXX

1110 XXXX

0XXX XXXX

XXXX XXXX

XXXX XXXX

0000 0001

0000 0000

0101 1110

32-bit IP address

48-bit MAC address

5 bits lost

25-bit MAC address prefix

Ă

Ă

23 bits

mapped