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Something I've always wondered about is MAC address exhaustion. Do hardware vendors keep track of MAC address allocation to their platforms, and then subsequently "reclaim" MAC addresses when a product goes EOL, and then re-use them on a new platform?

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MAC-48 has a maximum of 2^48 possible addresses which is somewhere in the neighborhood of 281 trillion different addresses(Compared to IPv4 which has around 4.2 billion). That said I've seen some studies and estimates saying that we won't have to worry about exhaustion until around 2100. The IEEE is one organization that supports this estimate. By that time we may not even being using MAC addresses anymore. Additionally there is another standard called EUI-64 which provides 2^64 possible addresses(24 being the OUI and 40 being the individual card number).

To answer your question about recycling, I don't imagine vendors do it nor that IEEE supports vendors doing it. When assigning local MAC addresses a network admin could do it but I can't imagine why you would want to.

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    It's also far less of an issue as a MAC only has to be locally unique. It's used at L2 (ethernet) to talk to locally connected hosts. And it's perverted into an IPv6 address (SLAAC) by adding another 80 bits to it, but your prefix is going to be globally unique anyway so duplicate MACs aren't a global problem. (read: I can use the same MAC as you as long as we aren't on the same ethernet segment.) – Ricky Beam May 21 '13 at 18:26
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    Since the vendor or manufacturer code (OUI) takes half of the bits, only 24 bits are left for unique addresses per OUI. This reduces each vendor pool to 16,777,216 NIC specific addresses. Many vendors have multiple OUIs. – generalnetworkerror May 22 '13 at 8:15
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    There are 2**22 OUI24s (two reserved flags), we've used 0.4% or 18k of them. 1 dollar buys you about 8900 MAC addresses. It would seem financially stupid to recycle them, as you'd end up with considerably more expensive support issues. OUIs are used for many things, not just MAC address (bluetooth, fiber channel, SFP/XFP manufactor code etc etc). – ytti Aug 11 '13 at 12:26
  • One bit is reserved to indicate multicast vs unicast messages. Another bit is reserved to indicate "globally unique" (OUI enforced) vs "locally administered" (MAC address is set according to a local scheme). – Craig McQueen Feb 8 '19 at 2:41
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I have wondered the same thing. I'm not sure if they do recycle the addresses, but unlike IP addresses, MAC addresses are only locally-significant to the broadcast domain they reside in.

Just as an example, the MAC address 0000.0c07.acXX is an HSRP group address where 'xx' is the HSRP group number. If you use the same group number on two different VLANs, the HSRP MAC address will be the same for both.

  • Yes, there are link-local multicast MAC addresses (thinking CDP, HSRP, GLBP, LLDP, sFlow, etc), but I'm referring to the traditional IEEE rules where MAC addresses should be unique to hardware. – John Jensen May 21 '13 at 16:27
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Western Digital did in the late 80's cards from Ireland and the USA occasionally used the same mac address, I know, as I had to debug a very weird issue which in the end came back to duplicate mac addresses on said cards

  • This is what I was going to say. Not with WD but with other manufacturers, we have seen duplicate MACs already. – jwbensley Apr 7 '18 at 14:06
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"MAC address exhaustion is unlikely to ever be a problem.

In the short term, 281 trillion addresses, even with a low fill rate, are a lot to work with. Even at 10% fill, that is 4k devices per person alive.

In the long term, if we arrive at a situation where all of the shirt buttons of the average Indian peasant are individually networked, MAC address shortage is easily addressed: MAC addresses only need to be unique locally, on a given LAN, and they don't need any particular structure. At the point when we are out of three-bytes prefixes to assign or recycle, we switch to the following algorithm:

Every device generates a random MAC on boot. Every device, when it detects MAC collision, switches to a randomly generated MAC.

Note that the collision will hardly ever happen, since the average number of devices on a LAN needed for a collision is 2^24, or nearly 17 million (cf What is the birthday problem?). Having 17 million hosts on the same broadcast domain is a bad idea, since Address Resolution Protocol traffic will be too high."

-Stanislav Shalunov

Source: http://www.quora.com/Will-MAC-Addresses-ever-be-exhausted

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