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Multicast Addressing

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After you do the math and map the last 23 bits, the MAC address becomes

01-00-5e-01-19-02. The easiest way to map layer 3 to layer 2 manually is to
do the math and make the binary conversion so you can see what the last 23
bits of the layer 3 IP address are. Once you have that number, all you have
to do is insert it into the MAC address and then calculate the remaining 3 hex
octet values. The first three octets will always be the same, 01-00-5e.

It is important that you spend time studying this procedure and the steps

needed to convert a layer 3 IP multicast address to a layer 2 MAC multicast
address.

There is one last method of determining the last 23 bits, but this method

will work only on some addresses. Keep in mind that the highest value you
can get in the second octet is 127 and still have it be included in the 23 bits
that will map to the MAC address. You know that the last 2 octets (3 and 4)
will map no matter what. So you will have 7 bits from the second octet and
a total of 16 bits from the last 2 octets for a total of 23 bits. Once your value
goes above 127 in the second octet, you will have to break down the octet
into binary so you can see the values of the first 7 fields.

Layer 3 to Layer 2 Overlap

After you have done a few of these conversions, you'll notice, or maybe you
already have, 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 16's value
position. This leaves 28 bits for host specification. However, by using only
23 bits of the layer 3 IP address, you leave 5 bits out of the mapping. This
causes an overlap of 2

, or 32 layer 3 addresses for every 1 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 9.7 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 5 bits. The last 23
bits are equivalent.

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