Multicast MAC Address

Multicast MAC Address
Multicast MAC addresses all start with the first 25 bits 01005E. The last 23
bits are the left-most bits from the IP address. Figure 7-4 shows how a MAC
address of 0100.5E40.0305 maps to a multicast IP address of 227.64.3.5.
Figure 7-4 Computing a Multicast MAC Address
IP 227.64.3.5 1 1 1 0
0 1 0 1
0 0 1 1
MAC 0100.5E40.0305 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 1 1 0
0 1 0 1
0 1 0 1
0 0 0 0
0 0 0 0
0 1 0 0
0 1 0 0
0 0 0 0
0 0 0 0
0 0 1 1
0 0 1 1
0 0 0 0
0 0 0 0
The first four bits of multicast IP addresses are always 1110, and the last 23
bits map to the MAC. That leaves five bits that are dropped. Remember that
this is an issue, because every multicast MAC maps to many multicast IPs!
Figure 7-5 shows how a MAC address of 0100.5E40.0305 could map to
several different multicast IP addresses. Notice that the first two octets can
vary in the IP addresses.
Figure 7-5 Overlapping Multicast MAC Addresses
CCNP BSCI
Chapter 7: IP Multicast [ 71 ]
IP 227.64.3.5 1 1 1 0 0 0 1 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 1 0 1
MAC 0100.5E40.0305 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 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 1 0 1