Layer 3 to Layer 2 Overlap

Layer 3 to Layer 2 Overlap
By the time you’ve done a few of these conversions, you’ll notice that there is a problem with
this conversion scheme. By not using all available bits for a Class D address, you cannot get an
accurate map of layer 3 to layer 2 addresses. If you look at properties of a Class D address, you
will see that the high order bit lies in the first octet and is in the 16s value position. This leaves
28 bits for host specification. However, by using only 23 bits of the layer 3 IP address, you leave
five bits out of the mapping. This causes an overlap of 2
5
, or 32 layer 3 addresses for every one
layer 2 address. With a ratio of 32:1, you can expect to see a significant amount of address ambiguity.
It is safe to say that any IP addresses that have the same values in the last 23 bits will map
to the same MAC multicast address.
For example, 224.0.1.1 and 225.128.1.1 map to the same MAC address. Figure 19.7 shows
why this is true. You can see that the bits that differ between 224.0.1.1 and 225.128.1.1 are all
within the lost five bits. The last 23 bits are equivalent.
The impact of this overlap can be significant. The overlap creates a window for multiple
multicast groups’ data to be forwarded to and processed by machines that didn’t intentionally subscribe
to the multiple groups. To give another example, a machine that subscribes to multicast group
224.2.127.254 would be given a MAC address of 01-00-5e-02-7f-fe. This host also processes packets
that come from multicast group 225.2.127.254 because the layer 2 MAC address is identical.