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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?

Arjan
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Justin Ethier
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    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 07 '11 at 20:01

3 Answers3

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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.

Matthias Braun
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Majenko
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  • Not only is it quite possible, it's done often (changing MAC address by software). Most cards even allow a MAC address to be reprogrammed several times over the lifetime of the card – jcolebrand Apr 07 '11 at 20:14
  • At a prior job where I installed loads of NICs we did come across an issue where the manufacture supplied a few of them with the same MAC. – Justin Apr 07 '11 at 21:59
  • I do not believe the last octets are randomly generated. There just is no *universal* rule governing how exactly it must be done, but some times we got some equipment (routers, computers, network cards) with consecutive numbers... –  Apr 08 '11 at 08:30
  • @Carlos Agreed. That's pretty much what I meant by randomly - there's no easy way for us mere mortals to predict them. They may be generated sequentially, but it's rare to be delivered consecutively numbered network cards. – Majenko Apr 08 '11 at 11:37
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    ...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. – Dave Sherohman Apr 08 '11 at 13:31
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    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! – Brian Knoblauch Apr 08 '11 at 13:35
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    The most annoying is old SUN systems where the quad ethernet cards had *one* MAC address for all four ports. – Majenko Apr 08 '11 at 14:02
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    @Xantec - Ahem, that's 16.7 million. – new123456 Jul 29 '11 at 04:35
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    @new123456 and so it is. All well its a little late to edit the comment now. – Xantec Jul 29 '11 at 11:27
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    @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 07 '14 at 20:52
  • @JaminGrey Sure. But if you're going to post figures, you should at least post them correctly ;) – new123456 Aug 07 '14 at 21:16
  • @new123456 True that! – Jamin Grey Aug 08 '14 at 23:07
  • This seem to me to be an issue of standardization and enforcement. Just as with GS1 Global and the UPC-A barcodes, there aren't technically any immediate regulatory ramifications on improper code issuance; the consequences naturally emerge once you scale up and enter retail at the national level with POS and EDD systems. However, there's no Weights-and-Measures department, so to speak, that handles non-compliance. My former employer quite literally had a hand-recorded journal with arbitrary numbering assignments notated, with absolutely no clue that the structuring included logic checks. – Arctiic Mar 05 '23 at 20:31
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u1686_grawity
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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.

Keltari
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