HP VPN Firewall Appliances Appendix Protocol Reference

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{ Scope—The Scope field contains four bits, which indicate the scope of the IPv6 internetwork for
which the multicast traffic is intended. Table 11 describes the values of the Scope field.
Table 11 Values of the Scope field
Value Meanin
g
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
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 41 IPv4-to-MAC address mapping
As shown in Figure 41, 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 Figure 42, the most signific
ant 16 bits of an IPv6 multic
ast MAC address are 0x3333.
The least significant 32 bits are the least significant 32 bits of a multicast IPv6 address.