Assigning a Layer 3 multicast address to a multicast group (application) automatically generates a Layer 2 multicast address. Figure 19-6 shows how a multicast MAC address is calculated from a Layer 3 multicast address. The MAC address is formed using an IEEE-registered OUI of 01005E, then a binary 0, and then the last 23 bits of the multicast IP address. The method is identical for Ethernet and Fiber Distributed Data Interface (FDDI).

Figure 19-6 Calculating a Multicast Destination MAC Address from a Multicast Destination IP Address

KEY Multicast

228.10.24.5

Multicast

01-00-5E

0 0 0 1 0 1 0 . 0 0 0 1 1 0 0 0 . 0 0 0 0 0 1 0 1 0

01-00-5E

0 0 0 0 1 0 1 0 . 0 0 0 1 1 0 0 0 . 0 0 0 0 0 1 0 1

Hex-

To understand the mechanics of this process, use the following six steps, which are referenced by number in Figure 19-6:

Step 1 Convert the IP address to binary. Notice the first 4 bits; they are always 1110 for any multicast IP address.

Step 2 Replace the first 4 bits 1110 of the IP address with the 6 hexadecimal digits (or 24 bits) 01-00-5E as multicast OUI, in the total space of 12 hexadecimal digits (or 48 bits) for a multicast MAC address.

Step 3 Replace the next 5 bits of the binary IP address with one binary 0 in the multicast MAC address space.

Step 4 Copy the last 23 bits of the binary IP address in the last 23-bit space of the multicast MAC address.

Step 5 Convert the last 24 bits of the multicast MAC address from binary to 6 hexadecimal digits.

Step 6 Combine the first 6 hexadecimal digits 01-00-5E with the last 6 hexadecimal digits, calculated in Step 5, to form a complete multicast MAC address of 12 hexadecimal digits.