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[eluser]CroNiX[/eluser]
PHP (let alone, CI) doesn't have a way to directly access a mac address, only the IP. I've seen examples by executing a shell command via PHP to get the MAC, but that's OS dependent and will only work on a local network, which you say you are using.

Doesn't each machine have it's own IP address? It seems that would be easier.

I don't follow your 2nd question.