Now we put MAC address into start script of the machine. It is important, because it allows machine to receive same address via DHCP, and also can affect internal working of operating system (i.e. linux guests can change name of network interface if mac changes). 

But if we move virtual machine from one host no another, mac conflict can happen.

So, we need MAC assigning algorithm, which minimize probability of the conflict, and method of conflict resolution on the first start of the machine on new host.