Excuse me. As a rule, a MAC address is a decent approximation of globally unique, certainly good enough to identify a single computer for advertising purposes.

Excuse me. As a rule, a MAC address is a decent approximation of globally unique, certainly good enough to identify a single computer for advertising purposes. If you have a MAC address and it's not globally unique, it's defective and out of spec. Which isn't to say duplicates never happen (ultra-cheap rip-off network cards, virtual machines, reprogramming your hardware manually, etc), but... there's a reason they go through the rigamarole of partitioning out different OUI blocks to different network equipment vendors.