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?
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.
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.
"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."