I have tried searching but got rather a lot of hits with the search term 'mac' and have not found a better one. I could also extend my code to allow the MAC address to be changed later (stored in EEPROM) and read at start up (as I have done with other configuration settings), but I thought this sort of issue can't be that unusual and wondered how others had dealt with it. While I could hand modify the sketch and burn it individually in to each box this seems a bit 'Heath-Robinson'. Given that these are supposed to be unique I feel this can't be a good thing, particularly on the same network segment. However this means if I put the same sketch in 2 boxes without modification they will have the same MAC address (so I could have 2 boxes with unique IP address from DHCP, but with the same MAC address). All the examples I have seen have a hard-coded MAC address in the sketch. The only thing I am wondering about is the MAC addressing.

I have built in commands to allow me to switch each unit to use DHCP or static IP addressing and to supply static IP addresses in case I want to set up each box and leave the same sketch (these values get saved in EEPROM and read in during setup along with various other configuration parameters). I am now looking that several of these may be on the same network attached to different instruments. Values for display are sent through UDP and it is all working nicely. The Arduino Uno R3 has an official Arduino Ethernet shield. I have a display box with a number of 7-segment 4-digit displays. My first arduino project is nearing completion (first stage at least).

How are mac addresses unique