Figure 1-6 IPv4-to-MAC address mapping
The high-order four bits of a multicast IPv4 address are 1110, indicating that this address is a multicast
address, and 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 MAC address.
Therefore, in Layer 2 multicast forwarding, a device may receive some multicast data addressed for
other IPv4 multicast groups, and such redundant data needs to be filtered by the upper layer.
IPv6 multicast MAC addresses
The high-order 16 bits of an IPv6 multicast MAC address are 0x3333, and the low-order 32 bits are the
low-order 32 bits of a multicast IPv6 address.
shows an example of mapping an IPv6
multicast address, FF1E::F30E:101, to a MAC address.
Figure 1-7 An example of IPv6-to-MAC address mapping