H3C Technologies H3C S12500-X Series Switches - Manual (Page 18)

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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—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
• 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
The most significant four bits of a multicast IPv4 address are 1110. 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, a device
might receive some unwanted multicast data at Layer 2 processing, which needs to be filtered by
the upper layer.
• IPv6 multicast MAC addresses:
As defined by IANA, the most significant 16 bits of an IPv6 multicast MAC address are 0x3333
as its address prefix. The least significant 32 bits are the least significant 32 bits of a multicast IPv6
address and are mapped to the remaining IPv6 multicast MAC address, so the problem of
duplicate IPv6-to-MAC address mapping also arises like IPv4-to-MAC address mapping.