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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?

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migrated from skeptics.stackexchange.com Apr 7 '11 at 19:49

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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 7 '11 at 20:01

2 Answers 2

up vote 23 down vote accepted

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.

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15  
1.7 million combinations is certainly not a trivial number, but it isn't a very high number either. Not when you take a large manufacturer (like Netgear or Linksys etc) that have been around for over a decade making new hardware with "unique" MACs all the time –  Xantec Apr 7 '11 at 22:29
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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 8 '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 8 '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 8 '11 at 14:02
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@Xantec - Ahem, that's 16.7 million. –  new123456 Jul 29 '11 at 4:35
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