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. Example 7-4 shows how a MAC address of 0100.5E40.0305 maps to a multicast IP address of FIGURE 7-4 Computing a Multicast MAC Address

io Mi

MAC 0100,5 £40.03051


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! Example 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

IP 227 64.3.5 MAC0100.5E40 0305 |

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