Ethernet Multicast Addressing

Ethernet Multicast Addressing

The datalink layer also requires additional functionality for mapping Class D IP addresses to ethernet MAC addresses. The procedure outlined in the RFC also applies to FDDI, but a procedure is not specified for a token ring. The mapping from multicast to token ring layer 2 addresses presented here are the implementation on Cisco routers. The ethernet and FDDI layer 3 to layer 2 address mapping is relatively straightforward. The low-order 23 bits of the IP multicast address replace the low-order 23 bits of the ethernet multicast address 01:00:5E:00:00:00, as shown in Figure 3-4.

Figure 3-4: Formation of the ethernet multicast address

As you can see in Figure 3-4, nine bits in the group IP address do not take place in the mapping, the upper byte, and the most significant bit of the next-to-upper byte. The upper four bits of the upper byte are always 1110 because these are all Class D IP addresses. This means that in reality there are only five bits that are not involved in the mapping. Whatever the value of these bits, the multicast ethernet address is the same. Because there are 32 possible combinations of five bits, the mapping is not unique. In the example in Figure 3-2, 31 other Class D IP addresses map to the same multicast ethernet address.

Table 3-1: Class D multicast IP addresses that map to the multicast ethernet address 01:00:5E:41:0A:9A

224.65.10.154

225.65.10.154

226.65.10.154

227.65.10.154

228.65.10.154

229.65.10.154

230.65.10.154

231.65.10.154

232.65.10.154

223.65.10.154

234.65.10.154

235.65.10.154

236.65.10.154

237.65.10.154

238.65.10.154

239.65.10.154

224.193.10.154

225.193.10.154

226.193.10.154

227.193.10.154

228.193.10.154

229.193.10.154

230.193.10.154

231.193.10.154

232.193.10.154

233.193.10.154

234.193.10.154

235.193.10.154

236.193.10.154

237.193.10.154

238.193.10.154

239.193.10.154

Let’s examine the most significant byte of the IP address, 225.65.10.154. The byte 225 is represented in binary as 1110 0001. The upper four bits do not change because they are always 1110 for a Class D IP multicast address. The lower four bits have a range of values from 0000 to 1111, so the decimal range of values for the upper byte is 224 (224 + 0) to 239 (224 + 15). The most significant bit of the next-to-upper byte can be either 0 or 1, so this byte can be either 65 (0 + 65) or 193 (65 + 128). The upper byte can take on 16 values and the next-to-upper byte can take on two values, so there is a total of 32 Class D IP multicast addresses (16 x 2) that map to the multicast ethernet address 01 00 5E 41 0A 9A, as listed in Table 3-1. A host implementation must not only examine the ethernet address of the received multicast ethernet frame at layer 2 but must also examine the multicast IP address at layer 3 to determine if the packet is destined for a group that the host has joined.

Exercise 3-1

Determine which Class D IP multicast addresses map to the multicast ethernet address 01:00:5E:5F:00:01.

Solution.  We need to add the low-order 23 bits of the multicast ethernet address to the partial IP address 1110 xxxxx000 0000 0000 0000 0000 0000, which gives us

1110 xxxxx101 1111 0000 0000 0000 0001

where xxxx x = 0000 0—1111 1.

With xxxx x = 0000 0, the IP address is 224.95.0.1.

With xxxx x = 1111 1, the IP address is 239.223.0.1.

The other 30 possible IP addresses are found by substituting xxxx x with 0000 1—1111 0.