According to Wikipedia:

A Media Access Control address (MAC address) is a unique identifier assigned to network interfaces for communications on the physical network segment

But how unique are MAC addresses on devices coming out of the factory? I seem to remember hearing a long time ago about how some manufacturers would reuse MAC addresses on their network cards. Does anyone have any hard facts one way or the other?

  • 3
    I've heard similar things about MACs being re-used. I'm kinda curious if there are any educated / sourced answers that come out of this.
    – peelman
    Apr 7, 2011 at 20:01

3 Answers 3


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 a lot, 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.

  • 2
    ...unless they're installed on the same board/chassis. I see sequential MACs far more often than not when dealing with devices that shipped from the manufacturer with multiple network interfaces on the same connection medium. Apr 8, 2011 at 13:31
  • 2
    They're supposed to be unique, but I've had to troubleshoot some bizarre issues on 2 occasions now where we got equipment with identical MACs. One was hardware embedded where somehow we got 2 NICs from the same manufacturer with identical MACs. Never supposed to happen, but it did. The second one was a piece of used equipment where someone had changed the MAC address on board to something that just happened to match an existing one in our network. VERY annoying, I hope they had a good reason. I can't say how much I HATE it when people twiddle their MACs for no good reason. DON'T TOUCH! Apr 8, 2011 at 13:35
  • 3
    The most annoying is old SUN systems where the quad ethernet cards had one MAC address for all four ports.
    – Majenko
    Apr 8, 2011 at 14:02
  • 8
    @Xantec - Ahem, that's 16.7 million.
    – new123456
    Jul 29, 2011 at 4:35
  • 3
    @new123456 The point Xantec made still stands though. Manufacturers make hundreds of millions of cards, so unless they get multiple 'manufacturer IDs' they'll almost have to repeat IDs.
    – Jamin Grey
    Aug 7, 2014 at 20:52

If you purchase cheap - generally Chinese - network adapters, you run a small risk of them not having unique MACs. I have actually seen a batch of NICs with all the same MACs. I had to download a utility from the manufacturer to modify them.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .