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
POINT IP Address
0 0 0 1 0 1 0 . 0 0 0 1 1 0 0 0 . 0 0 0 0 0 1 0 1 0
0 0 0 0 1 0 1 0 . 0 0 0 1 1 0 0 0 . 0 0 0 0 0 1 0 1
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.
Unfortunately, this method does not provide a unique multicast MAC address for each multicast IP address, because only the last 23 bits of the IP address are mapped to the MAC address. For example, the IP address 220.127.116.11 produces exactly the same MAC address, 0x01-00-5E-0A-18-05, as 18.104.22.168. In fact, because 5 bits from the IP address are always mapped to 0, 25 (32) different class D IP addresses produce exactly the same MAC address. IETF points out that the chances of two multicast applications on the same LAN producing the same MAC address are very low. If it happens accidentally, a packet from a different IP multicast application can be identified at Layer 3 and discarded; however, network administrators should be careful when they implement multicast applications so that they can avoid using IP addresses that produce identical MAC addresses.
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