Multicast Architecture 547
Figure 165 IPv6 multicast format
■ 0xFF: 8 bits, indicating that this address is an IPv6 multicast address.
■ Flags: 4 bits, of which the high-order flag is reserved and set to 0; the definition
and usage of the second bit can be found in RFC 3956; and definition and
usage of the third bit can be found in RFC 3306; the low-order bit is the
Transient (T) flag. When set to 0, the T flag indicates a permanently-assigned
multicast address assigned by IANA; when set to 1, the T flag indicates a
transient, or dynamically assigned multicast address.
■ 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 54.
■ Reserved: 80 bits, all set to 0 currently.
■ Group ID: 112 bits, identifying the multicast group. For details about this field,
refer to RFC 3306.
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 0x0, 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 166.
Table 54 Values of the Scope field
Value Meaning
0, 3, F Reserved
1 Node-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
Group ID (112 bits)
0xFF Flags Scope
0 7 11 15 31
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