In order to function properly, MAC addresses need to be unique *on the 
network segment where they appear*.  Since they don't propagate past 
routers, it doesn't matter if the same MAC address appears on some other 
network segment.

  The process of assigning MAC addresses to be burned into hardware should 
assure that such addresses are unique in the world, and so under most 
circumstances this achieves the uniqueness actually required, and then some.
However, many devices include a way to override the hardware address, and 
this is sometimes useful.  For instance, a SOHO router will often allow the 
"cloning" of the address of one of the local computers on its "WAN" 
interface that faces the ISP.

  Someone here wrote that this uniqueness didn't matter as long as the layer 
3 IP addresses were unique, but in fact all those addresses do is locate the 
subnet and map to a MAC address -- if the MAC addresses aren't unique, it 
won't matter if the IP addresses are.  There's an important mechanism -- 
which users can usually ignore! -- where a network member sends out a "Hey, 
whoever has IP address x.x.x.x, please send me your MAC address" to do the 
mapping.

  DHCP, if used, will only hand out one IP address at a time to each MAC 
address that asks.  So in the ordinary case, if two devices have the same 
MAC address, they will be given the same IP address too in the belief that 
they are a single device.

  I cannot imagine a valid reason for a BIOS update to overwrite the MAC 
address "burned in" in the hardware.  Certainly if there is some reason that 
must be done, I agree that the update will probably set all targets to the 
same address, and so if you have more than one on a segment, you may need to 
override all but one of them to give them unique values.

David Gillett, CCNP

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