Are MAC addresses unique when coming out of the factory?

There are 2⁴⁸ or 281 474 976 710 656 different potential combinations.

They are reasonably unique.

• The first 3 octets define the manufacturer.
• The last 3 octets are usually generated at the time of PROM burning. It's up to the manufacturer how they do this.

That obviously gives 16 777 215 possible unique MAC addresses per manufacturer. That's quite alot, so the manufacturer shouldn't re-use one. Some are lazy though, and don't check if they have already allocated a MAC address.

It is quite often possible to change the MAC address using software, so if you do get a duplicate you can map around it.

• Operating systems can override the MAC address in packets. The user can configure it. As another example, a virtual machine would send packets using its own address.

• Manufacturers may reuse addresses, sometimes unintentionally.