Location of MAC address

Somewhere on an (ethernet) Arduino there is storage for the MAC address, which
is generally assigned to the hardware on build. Does anyone know where it is,
and if it’s possible to get at it via software if it’s been lost? My guess is that its
somewhere on the W5100 chip, but it might be in a ROM somewhere.

Thanks - Will

Does anyone know where it is, and if it's possible to get at it via software if it's been lost?

The MAC address is defined by the sketch running on the Arduino. It is not a value that is burned into the hardware anywhere.

The MAC address simply needs to be unique on your network. If you removed the sticker from the shield, and lost it, simply make up a new value that is not the same as any other device on your network.

mac addresses in your local network can be found (windows) with tools like - http://www.radmin.com/products/ipscanner/ -

or the command "arp -a" on a commandline