Originally Posted by elias_vc
Now, the question is why; why can I change it? I mean, isn't that address supposed to be unique or something?
Well, yes, its supposed to be unique and you will get into trouble if it isn't unique within a collision domain.
My guess is that shortly after the first person made the mistake of trying to filter net access by MAC address, someone came up with a hack to get around it, although it might also be the case that some of the supposedly unique-from-the-factory addresses weren't as unique as they should have been and a hack was needed to get around that.