1-9
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, F
Reserved
1 Interface-local
scope
2 Link-local
scope
3 Subnet-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
.
