Every computer's network interface has a unique identifer called a MAC address. It is a 12 digit hexadecimal number usually delimted by colons, e.g. ab:01:23:ef:45:dc. Both wired and wireless network interfaces have these addresses.
Finding the Ethernet MAC Address
For PCs running Windows 7 and earlier, Go to Start Menu > Programs > Accessories > DOS Command Prompt. Type cmd. For PCs running Windows 8 and later, launch the "Command" program by searching for it in your applications list. When the command window appears, type ipconfig /all. The Physical Address value for your ethernet card is your MAC address. Make sure you're looking at your ethernet interface and not your wireless interface.
Apple MacOS X
From the Desktop
Go to the Apple menu > System Preferences > Network (under "Internet and Wireless"). Click on the Advanced button and then the Hardware tab. The MAC address is listed there.
From the Terminal
Type "networksetup -listallhardwareports". The "Ethernet Address" field for the ethernet hardware port is your MAC address.
Type ip link in a terminal window. The link/ether field associated with your ethernet interface is your MAC address.