Multicast Protocols - 3Com E4500-24 Cli Configuration Manual

Class D address range
224.0.0.16
224.0.0.17
224.0.0.18
224.0.0.19 to 224.0.0.255
Like having reserved the private network segment 10.0.0.0/8 for unicast, IANA has also reserved the
network segment 239.0.0.0/8 for multicast. These are administratively scoped addresses. With the
administratively scoped addresses, you can define the range of multicast domains flexibly to isolate IP
addresses between different multicast domains, so that the same multicast address can be used in
different multicast domains without causing collisions.
Ethernet multicast MAC address
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.
describes the mapping relationship:
Figure 1-4 Multicast address mapping
32-bit IP address
48-bit MAC address
0000 0001
25-bit MAC address prefix
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.

Multicast Protocols

XXXX X
1110 XXXX
0000 0000 0101 1110
The specified subnetwork bandwidth
management (SBM)
All SBMS
Virtual Router Redundancy Protocol (VRRP)
Other protocols
5 bits lost
XXXX XXXX XXXX XXXX
23 bits
...
mapped
0XXX XXXX XXXX XXXX
1-8
Description
Figure 1-4
XXXX XXXX
...
XXXX XXXX