8
Bit Descri
tion
P
• When set to 0, it indicates that this address is an IPv6 multicast address not based on
a unicast prefix.
• When set to 1, it indicates that this address is an IPv6 multicast address based on a
unicast prefix—the T bit must also be set to 1.
T
• When set to 0, it indicates that this address is an IPv6 multicast address permanently-
assigned by IANA.
• When set to 1, it indicates that this address is a transient, or dynamically assigned
IPv6 multicast address.
• Scope—The Scope field contains four bits, which indicate the scope of the IPv6 internetwork for
which the multicast traffic is intended.
Table 5 Values of the Scope field
Value Meanin
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
• Group ID—The Group ID field contains 112 bits. It uniquely identifies an IPv6 multicast group in the
scope that the Scope field defines.
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, 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 most-significant 24 bits of an IPv4 multicast MAC address are 0x01005E. Bit
25 is 0, and the least-significant 23 bits are the least-significant 23 bits of a multicast IPv4 address.
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