UMN:CLI User Manual
V5808
402
IP Multicast to Ethernet/FDDI MAC Address Mapping
All IP multicast frames use MAC layer addresses beginning with the 24-bit prefix of
0x0100.5Exx.xxxx. With only half of these MAC addresses available for use by IP
Multicast, 23 bits of MAC address space are available for mapping L3 IP multicast
addresses into L2 MAC addresses.
As there are 28 bits (32 – 4 Class D prefix) of unique address space for an IP multicast
address and only 23 bits are mapped into the MAC address, there are 5 bits of overlap
(28 – 23). These 5 bits represent 25 = 32 addresses. Therefore, there is a 32:1 overlap of
IP addresses to MAC addresses – 32 IP multicast addresses are mapped to the same
MAC multicast address. As an example, below lists all the IP multicast addresses that are
mapped to the same MAC multicast address of 0x0100.5E01.0101.
The 32:1 address mapping ambiguity can lead to over subscription problem at hosts. You
must consider it when you allocate IP multicast addresses for network applications to
prevent unexpected behavior.
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