H3C S3100 8C SI Operation Manual page 313

Operation Manual – Multicast
H3C S3100 Series Ethernet Switches
Class D address
224.0.0.18
224.0.0.19 to
224.0.0.255
Note:
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.
II. 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.
48-bit MAC address
0000 0001
Figure 1-4 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.
range
Virtual router redundancy protocol (VRRP)
Other protocols
Figure 1-4 describes the mapping relationship:
32-bit IP address
0000 0000
25-bit MAC address prefix
Description
5 bits lost
XXXX X
1110 XXXX XXXX XXXX
...
0101 1110 0XXX XXXX
1-9
Chapter 1 Multicast Overview
XXXX XXXX XXXX XXXX
23 bits
...
mapped
XXXX XXXX XXXX XXXX