MAC address recycling?

  • 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?

  • Carlo M

    Carlo M Correct answer

    8 years ago

    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.

    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_.)

    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.

    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).

    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).

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Content dated before 7/24/2021 11:53 AM