3com WX3000 Manuel D’Utilisation

 

1-9 

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. 

 

describes the mapping relationship: 

Figure 1-4 Multicast address mapping 

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

…

…

23 bits

mapped

 

 

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.  

 

z 

Generally, we refer to IP multicast working at the network layer as Layer 3 multicast and the 

corresponding multicast protocols as Layer 3 multicast protocols, which include IGMP, PIM, and 

MSDP; we refer to IP multicast working at the data link layer as Layer 2 multicast and the 

corresponding multicast protocols as Layer 2 multicast protocols, which include IGMP Snooping.  

z 

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 3 multicast protocols include multicast group management protocols and multicast routing 

protocols. 

 describes where these multicast protocols are in a network.