Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 258
Mapping to an Ethernet or IEEE 802.X MAC Address

Network interface cards are not interested in any type of Layer 3 addressing. NICs receive and transmit data on the network using MAC (Media Access Control) or hardware addresses. Therefore, some type of mapping must be used to map an IP multicast address to a MAC address. But the NIC plays an important role in receiving multicast packets. Somehow, there has to be a MAC address for multicast, and one for all multicast packets is not efficient. You should note that up to 32 different IP multicast groups may be converted to the same MAC address. The upper 5 bits of the IP address are ignored. It looks like the upper 9 bits, but the first four bits of a Class D address are always 1110 (which converts to a 224 in decimal, the starting number for Class D addresses), and since 9 bits are displaced in this procedure, only the next 5 bits are really ignored. If you read through RFC 1700 you will see that most of the assigned addresses will not be affected by this procedure.

When used on an Ethernet or IEEE 802 network, the 23 low-order bits of the IP multicast address are placed in the low-order 23 bits of the Ethernet or IEEE 802 net multicast address. The IANA has been allocated a reserved block of MAC layer addresses. Therefore, a multicast MAC address always begins with 01-00-5E (hex).

For example, refer to the slide. The IP multicast address 224.0.1.88 is mapped into a MAC address (converted to hex). First, the IP address must be converted to hex (it is usually written in dotted decimal notation as shown). The address 224 is E0 in hex, 0 is 00 in hex, 1 is 01 in hex, and 88 is 58 in hex. However, only the low-order 23 bits are used. Therefore, the IP address of 224.0.1.88 converted to a MAC address is 01-00-5E-00-01-88.

Hex
     01-00-5E-00-00-00
Binary

0                           23                           47
|                           |                             |
0000 0001 0000 0000 0101 1110 0xxx xxxx xxxx xxxx xxxx xxxx
        |                     |
 Multicast bit                0=Internet multicast
                            1 = Assigned by IANA for other uses

In order for the NIC card to receive or transmit multicast packets, the following functions must be invoked to place the multicast address in the NIC card and to remove it.

JoinLocalGroup (mapped group address): Allows the link layer to receive multicast packets for a particular host group.

LeaveLocalGroup (mapped group address): Allows the link layer to stop receiving multicast packets for a particular host group.

The mapped group address is the MAC address that is mapped from the host group multicast address.