STMU5 best UID to MAC hashing/compression methods

[danyavin]

I’d like to create a mac address using the STM32U5’s 96-bit UID. 
since first byte I’ll use as constant, i need 40 bits to create a unique mac for my devices.
i wonder what’s the best way to create a mac with the least change of collision.
if i use 32 bit wafer coordinates, and 8 bit wafer number for example, how likely am i to get a collision, opposed to other methods?
thank you

[MClar.3]

This all depends on how many things you are creating and what you are doing with them. 

If it’s just a personal project for use on your own network, then I’d guess you’ve got better chances of winning the lottery than getting a collision from a randomly generated number.

If it’s something you need to be able to connect to another network, Atmel sell some EEPROMs with pre-programmed unique MAC addresses - Look here.

If you’re building a large number of things, you probably need to buy a block from the IEEE - Look here.

[danyavin]

thank you for the replay. its for a device my company already started manufacturing. the devices will sit on our own local network using a switch, with a few devices per network. for now I’ve implemented the MAC from the wafer number and X-Y coordinates.
you think the chances of collision are slim enough, or would you recommend working with the RNG?
 

[MClar.3]

Was typing out a reply, and found something way better!

There are things called locally administered addresses - The MAC equivalent of private IP ranges.

There are four huge blocks available:

x2:xx:xx:xx:xx:xx, x6:xx:xx:xx:xx:xx, xa:xx:xx:xx:xx:xx, and xe:xx:xx:xx:xx:xx

I’d break this down into a smaller more manageable block, so you could have a prefix like 02:de:ad:be:ef: and then 256 addresses under this, so 02:de:ad:be:ef:00 to 02:de:ad:be:ef:ff 

Or, if you’ve got wafer coordinates, use a smaller prefix, and a 16 bit number for x-y, so 02:00:xx:xx:yy:yy.

Same caveat applies, though, they shouldn’t be placed anywhere outside your own network. 

[hardikag]

Is there an ST-recommended way to derive a production MAC from the UID, or is the expectation that

anyone shipping in volume just registers their own IEEE OUI? And is there an application

note or wiki page on production MAC provisioning for STM32 Ethernet designs? I looked and

couldn't find one.

Thanks.