8
{
Scope
—Contains four bits, which indicate the scope of the IPv6 internetwork for which the
multicast traffic is intended.
describes the values of the Scope field.
Table 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.
{
Group ID
—Contains 112 bits. It uniquely identifies an IPv6 multicast group in the scope that the
Scope field defines.
Ethernet multicast MAC addresses
A multicast MAC address identifies a group of receivers at the data link layer.
•
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 other 23 bits are the least significant 23 bits of a multicast IPv4 address.
Figure 6
IPv4-to-MAC address mapping
As shown in
, the most significant four bits of a multicast IPv4 address are 1110, which
means that this address is a multicast address. Only 23 bits of the remaining 28 bits are mapped
to a MAC address, so five bits of the multicast IPv4 address are lost. As a result, 32 multicast IPv4
addresses map to the same IPv4 multicast MAC address. Therefore, in Layer 2 multicast
forwarding, a switch might receive some multicast data destined for other IPv4 multicast groups.
The upper layer must filter such redundant data.
•
IPv6 multicast MAC addresses:
As shown in
, the most significant 16 bits of an IPv6 multicast MAC address are 0x3333.
The least significant 32 bits are the least significant 32 bits of a multicast IPv6 address.
