Mapping IP Multicast Addresses to Ethernet

Ethernet frames have a 48-bit destination address field. To avoid invoking the Address Resolution Protocol (ARP) to map multicast IP addresses to Ethernet addresses, the IANA designated a range of Ethernet addresses for multicast. The lower 23 bits of the Class D address are mapped into a block of Ethernet addresses that have been reserved for multicast. This block includes addresses in the range 00:00:5e:00:00:00 through 00:00:5e:ff:ff:ff. The IANA allocates half of this block for multicast addresses. Given that the first byte of any Ethernet address must be 01 to specify a multicast address, the Ethernet addresses corresponding to IP multicasting are in the range 01:00:5e:00:00:00 through 01:00:5e:7f:ff:ff.

NOTE

Multicast MAC and IP addresses only belong in destination addresses.

The prefix 01-00-5e identifies the frame as multicast; the next bit is always 0, leaving only 23 bits for the multicast address. Because IP multicast groups are 28-bits long, the mapping cannot be one-to-one. Only the 23 least-significant bits of the IP multicast group are placed in the frame. The remaining five high-order bits are ignored, resulting in 32 different multicast groups being mapped to the same Ethernet address. For example, if the IP multicast group address is 224.10.8.5, the destination MAC address becomes 01-00-5E-0A-8-5.

When mapping multicast addresses to MAC addresses, all 28 bits of the IP multicast cannot be mapped to the available 23 bits of MAC address space, meaning that five bits of address are lost. This also means that 32, or 25, addresses can be ambiguous when mapped. Therefore, each IP multicast MAC address is capable of representing 32 IP multicast addresses.

Figure 10-6 illustrates a multicast group address being mapped into an IEEE-802 multicast address. In multicast addressing, the high-order nine bits of the IP address are not mapped into the MAC-layer multicast address. In the example in Figure 10-6, the mapping places the low-order 23 bits of the IP multicast group ID into the low order 23 bits of the IEEE-802 multicast address.

Figure 10-6 Mapping Multicast Address to Ethernet

IP Multicast Address 0 7 8 15 16 23 24 31

1110

224

Not <—►

01

00

Used 5E

00000001

00000000

0101

1110

0

Low-Order 23 Bits of Multicast <->-

Group ID Copied to Ethernet Address

0

48-Bit Ethernet Address

Continue reading here: Distribution Trees

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