IP multicast mapped to MAC multicast 635

IP multicast mapped to MAC multicast
0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 1 0 1 1 1 1 0 0
1 1 1 0 x x x x x y y y y y y y y y y y y y y y y y y y y y y y
01005e
High order bit
24-bit MAC prefix
OUI (Organizational Unique Identifier)
23 bits for layer 3 mapping
5 bits lost from mapping
the last 23 bits
Class D mask
Using Multicast Addressing
593
FIGURE 1 9 . 5
Example 1 for mapping IP multicast to MAC multicast addresses
Convert the octets from decimal into binary so you can get a clear picture of what the last
23 bits are. Here you would see the following address (the last 23 bits are indicated in bold font):
11100001.0
0000001.00011001.00000010
. Also, as you can see, Figure 19.6 depicts the last
23 bits that are mapped to the free spaces of the multicast MAC address. After the mapping has
occurred in binary, convert the binary value to hex and you have the new MAC multicast address.
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. After 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 are always 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 works 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 two octets (3 and 4) will map no matter what. So you have 7 bits from the second octet,
and 16 bits from the last two octets, for a total of 23 bits. After your value goes above 127
in the second octet, you have to break down the octet into binary so you can see the values
of the first seven fields.
0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 1 0 1 1 1 1 0 0
1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1
01005e
High order bit
24-bit MAC prefix
OUI (Organizational Unique Identifier)
23 bits for layer 3 mapping
0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 1 0 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1

Convert the octets from decimal into binary so you can get a clear picture of what the last
23 bits are. Here you would see the following address (the last 23 bits are indicated in bold font):
11100001.0
0000001.00011001.00000010
. Also, as you can see, Figure 19.6 depicts the last
23 bits that are mapped to the free spaces of the multicast MAC address. After the mapping has
occurred in binary, convert the binary value to hex and you have the new MAC multicast address.
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. After 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 are always 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 works 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 two octets (3 and 4) will map no matter what. So you have 7 bits from the second octet,
and 16 bits from the last two octets, for a total of 23 bits. After your value goes above 127
in the second octet, you have to break down the octet into binary so you can see the values
of the first seven fields.