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. Figure 1-4
describes the mapping relationship:
Figure 1-4 Multicast address mapping
XXXX X
XXXX XXXX XXXX XXXX XXXX XXXX1110 XXXX
0XXX XXXX XXXX XXXX XXXX XXXX0000 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 bit s
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.
Multicast Protocols 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
Layer 3 multicast protocols include multicast group management protocols and multicast routing
protocols. Figure 1-5 describes where these multicast protocols are in a network.