The mac address is a unique number associated with each NIC. These are supposed to be unique throughout the world.
This creates something of a dilemma for a VM system. I discovered recently that when VMWare asks you whether you 1) Copied a VM or 2) Moved it, if you choose 1) he changes the mac address of the virtual NIC. So that would cause two systems to stop communicating. On the other hand if you cloned a VM and chose 2) with the idea that the two would be able communicate, this would fail because the two systems would have identical macs. At least I think this is the case.