Mapping IPv4 Multicast Addresses to Ethernet Addresses

For multicast to work properly in IP-over-Ethernet, each IP multicast address must be mapped to a corresponding Ethernet multicast address.

The IANA owns an Ethernet OUI (01:00:5e), half of which has been reserved for this purpose.

Problem:

However, the address space for IP multicast addresses is 28 bits in length, which is larger than the range of addresses reserved in the Ethernet OUI, as only 23 bits have been allocated to this.

To deal with this issue, only the lower 23 bits of each IP multicast address are used in this address mapping process.