Ryan Carretta Tue, 06/03/2008 - 02:05

Multicast MAC addresses always have the low-order bit of the high-order octet set to 1.

That is, the second hexadecimal digit from the left in a multicast address will always be odd (1, 3, 5, 7, 9, B, D, F).

paul.matthews Tue, 06/03/2008 - 03:12

A little more info on what multicast you are using would help - basically the last bit of the first byte indicates an address is multicast - eg 08000d123456 is unicast 09000d123456 is multicast.

There are lots of different uses for multicast at L2 - some systems use them for finding peripherals - eg printers etc. some devices use them as a proprietary alternative to Bootp etc. They will tend to use the manufacturers prefix as the base address, the made up ones here are the old ICL range.

Then there is multicast mac addressing and mapping to IP multicast addresses, where the whole MAC address maps to a number of IP multicast addresses.

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