2-9
Bit Description
T
z When set to 0, it indicates that this address is an IPv6 multicast address
permanently-assigned by IANA
z When set to 1, it indicates that this address is a transient, or dynamically
assigned IPv6 multicast address
z Scope: 4 bits, indicating the scope of the IPv6 internetwork for which the multicast traffic is
intended. Possible values of this field are given in
Table 1-5.
Table 1-5 Values of the Scope field
Value Meaning
0, 3, F Reserved
1 Interface-local scope
2 Link-local scope
4 Admin-local scope
5 Site-local scope
6, 7, 9 through D Unassigned
8 Organization-local scope
E Global scope
z 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 addresses
When a unicast IP packet is transmitted over Ethernet, the destination MAC address is the MAC
address of the receiver. When a multicast packet is transmitted over Ethernet, however, the destination
address is a multicast MAC address because the packet is directed to a group formed by a number of
receivers, rather than to one specific receiver.
1) IPv4 multicast MAC addresses
As defined by IANA, the high-order 24 bits of an IPv4 multicast MAC address are 0x01005E, bit 25 is 0,
and the low-order 23 bits are the low-order 23 bits of a multicast IPv4 address. The IPv4-to-MAC
mapping relation is shown in
Figure 1-6.
Figure 1-6 IPv4-to-MAC address mapping
XXXX X
XXXX XXXX XXXX XXXX XXXX XXXX1110 XXXX
0XXX XXXX XXXX XXXX XXXX XXXX0000 0001 0000 0000 0101 1110
32-bit IPv4 address
48-bit MAC address
5 bits lost
25-bit MAC address prefix
…
23 bits
mapped
…
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