Speaking of unique identifiers: when I was like 13, my parents bought new ethernet cards so that all the kids would have their own computer. Two adjacent ones on the shelf that happened to have the same MAC address. Not sure how they figured it out (probably just looking for anything wrong and noticing the MAC addresses were identical), but the network behavior was interesting. Basically if you mashed F5 on a webpage the router would think you were the proper owner of all the network traffic for both computers, and it would disconnect your sibling from whatever they were playing at the time.