Or at least they're unique on a single machine... except for the various OSes and hardware platforms that have deliberately made all MAC addresses the same for a given machine.
Way back in the land before time, I acquired a number of old 486s and ISA NICs. Being a good Slashdot nerd, I of course thought "lets make a Beowulf cluster of these!"
So I've got the cluster set up, and it's mostly working. There's two nodes though that behave funny. Imagine my surprise when I try pinging one of them and get two responses per ping! Sure enough, two ISA NICs pulled randomly out of a box of garbage indeed had identical MACs. I learned a lot about IP and the MAC layer from that project!
Or at least they're unique on a single machine... except for the various OSes and hardware platforms that have deliberately made all MAC addresses the same for a given machine.