Originally Posted by arashpartow
Actually they are
But, it is possible to change the MAC address (see the rest of the article)
Plus, for some old NICs there was duplication of MAC addresses. If I remember correctly it was for NIC that were designed for different topologies, token ring and bus. It caused a problem when people bridged the two networks.
That said, MAC addresses should be unique if they are on the same network. (I think that unique MAC addresses is a requirement for ethernet)