In Windows 2000, NT or XP
Go to 'Start' and choose 'run'. type 'command' in the box and click 'OK'. A terminal window will pop up. At the prompt type 'ipconfig -all' and hit enter on your keyboard. a bunch of information will scroll past including the Physical Address .
In Macintosh OS X
Double click on the System Preferences icon or click on the apple and double click on System Preferences. Click on Network. The Ethernet Address is listed at the bottom of the window.