H3C S3100 Series Operation Manual page 282

E
Group ID: 112 bits, IPv6 multicast group identifier that uniquely identifies an IPv6 multicast group in the
scope defined by the Scope field.
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.
1) IPv4 multicast MAC addresses
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 1-6 Multicast address mapping
32-bit IP address
48-bit MAC address
0000 0001 0000 0000
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.
2) IPv6 multicast MAC addresses
The high-order 16 bits of an IPv6 multicast MAC address are 0x3333, and the low-order 32 bits are the
low-order 32 bits of a multicast IPv6 address.
multicast address, FF1E::F30E:101, to a MAC address.
Figure 1-7 An example of IPv6-to-MAC address mapping
Value
XXXX X
1110 XXXX XXXX XXXX
0101 1110 0XXX XXXX
Global scope
5 bits lost
XXXX XXXX
23 bits
...
mapped
XXXX XXXX
Figure 1-7 shows an example of mapping an IPv6
1-9
Meaning
Figure 1-6
XXXX XXXX
...
XXXX XXXX