A MAC (Media Access Control) address is the 12 digit hexadecimal address assigned by a manufacturer to every Ethernet device made. Ethernet adapter address and Network Interface Card (NIC) address are terms that mean the same thing.

How to find a hardware address - Windows Operating Systems

For Windows 2000 and XP machines:

Go to "Start > All Programs > Programs > Accessories > Command Prompt"
Type "ipconfig /all"
Press "Enter"

For Windows 95 and 98 machines:

Go to "Start"
Go to "Run"
Type winipcfg
Click "OK"

For Windows NT machines:

Go to "Start"
Go to "Programs"
Go to MS DOS Command Prompt
Type ipconfig/all
Press "Enter"

How to find a hardware address - Macs

For Mac TCP:

Click on the Apple icon
Go to "Control Panels"
Select "Mac TCP"
Click on "Ethernet" while holding the Option key

For MacOS running Open Transport:

Go to "Control Panels"
Go to "Control Panels"
Select "TCP/IP"
Press Command "l"

If the above methods fail (for Macs):

Go to the Apple menu
Go to Control Panels
Select AppleTalk
Make sure that ethernet is selected
Go to Edit menu and select "usermode"
Choose "advanced"
Go back to the Appletalk control panel and click on the info button. The MacAddress should be listed under "Hardware Address".
From the TCPIP Control Panel you can change user mode to advanced and click on the info button. This will again tell you the macaddress, listed as "Hardware Address".