H3C Technologies H3C S3600 Series Switches - Manual (Page 383)

1-9
Class D address range Description
224.0.0.18 Virtual Router Redundancy Protocol (VRRP)
224.0.0.19 to 224.0.0.255 Other protocols
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. Figure 1-4
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.
Multicast Protocols