Mapping IP Multicast Addresses to Ethernet and FDDI MAC Addresses

To support IP multicasting, the Internet authorities have reserved the multicast address range of 01-00-5E-00-00-00 to 01-00-5E-7F-FF-FF for Ethernet and Fiber Distributed Data Interface (FDDI) media access control (MAC) addresses. As shown in Figure 4.1, the high order 25 bits of the 48-bit MAC address are fixed and the low order 23 bits are variable.

To map an IP multicast address to a MAC-layer multicast address, the low order 23 bits of the IP multicast address are mapped directly to the low order 23 bits in the MAC-layer multicast address. Because the first 4 bits of an IP multicast address are fixed according to the class D convention, there are 5 bits in the IP multicast address that do not map to the MAC-layer multicast address. Therefore, it is possible for a host to receive MAC-layer multicast packets for groups to which it does not belong. However, these packets are dropped by IP once the destination IP address is determined.

For example, the multicast address 224.192.16.1 becomes 01-00-5E-40-10-01. To use the 23 low order bits, the first octet is not used, and only the last 7 bits of the second octet is used. The third and fourth octets are converted directly to hexadecimal numbers. The second octet, 192 in binary is 11000000. If you drop the high order bit, it becomes 1000000 or 64 (in decimal), or 0x40 (in hexadecimal). For the next octet, 16 in hexadecimal is 0x10. For the last octet, 1 in hexadecimal is 0x01. Therefore, the MAC address corresponding to 224.192.16.1 becomes 01-00-5E-40-10-01.

The 25th bit is always zero.