Windows NT/2000/ME/XP
Click on your Start button, and then click on Run...

This will bring up a small window where you can enter a line of text. Type in command and click the OK button.

A black window called C:\Winnt\system32\command.com should pop up. If you get an error message instead, try typing in "c:\winnt\command.com". 

Type in ipconfig /all and hit return. It should come up with a list like the window below. 

Look for the line called Physical Address. Your MAC address is the set of numbers and/or letters following Physical Address.  Please note the MAC address in the picture above will be different from what your computer will display.

Windows 95/98
Click on your Start button, and then click on Run...

This will bring up a small window where you can enter a line of text. Type in winipcfg and click the OK button.

A window called IP Configuration should pop up. If you get an error message instead, you probably don't have the TCP/IP protocol installed--click here for instructions on how to install the TCP/IP protocol.

Click on the tiny triangle on the right end of the pop-down menu. You may see several choices, only one of which corresponds to your Ethernet card. The choice you want probably has the word "ether" or "Ethernet" in it. If an item has Dial-Up, VPN, PPP or AOL in it, it is NOT the choice you want.

By process of elimination, you should be able to figure out which choice corresponds to your Ethernet card. Your MAC address is the set of numbers and/or letters in the Adapter Address box. The other boxes are unimportant.

Macintosh OS 8.5+
Click on your Apple menu, and select Apple System Profiler.

This should bring up the Apple System Profiler program. Click on the little blue triangle next to Network overview. This will reveal some more text and triangles. Click on the triangle next to AppleTalk. This will reveal some more text. Your Ethernet Hardware Address is the numbers and letters to the right of where it says "Hardware Address:"

Macintosh OS 8.0 - 8.5
Click on your Apple menu, and select Control Panels and then TCP/IP

This should bring up your TCP/IP control panel. Make sure your Ethernet port is selected in the pop-up menu at the top of the TCP/IP control panel. Click the Info button near the bottom-left corner of the window.

This should bring up the TCP/IP Info window. The set of numbers and/or letters in the Hardware address: section is your Ethernet Hardware Address. Your Macintosh must be connected to a live Ethernet jack for this number to show up.