Multicast Addressing
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F I G U R E 9 . 4
IP multicast mapped to MAC multicast
Let's look at some examples for mapping layer 3 multicast addresses to
layer 2 multicast addresses. A local IP multicast address is 224.0.0.1. Refer
to Figure 9.5 to see how this is mapped. The conversion from binary to hex
reveals the MAC multicast address. The prefix was 01-00-5e. The last 23
bits, including the high order bit, give you 00-00-01. Put them together and
you get 01-00-5e-00-00-01 as the MAC address.
F I G U R E 9 . 5
Example #1 for mapping IP multicast to MAC multicast addresses
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
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
01-00-5e-00-00-01
224.0.0.1
Final MAC
multicast address
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