Your iMac's MAC (Media Access Control) address is a 12-digit identification number applied by the computer's manufacturer for the purposes of identifying a port or adaptor on a LAN (Local Area Network). If you've found yourself needing to troubleshoot your iMac's network connections, and don't know where to find this number--don't worry--there's no need to disassemble your iMac and start digging. All it will take is a moment of your time and a few mouse clicks.
Access the main desktop menu on your iMac. Highlight and click the "System Preferences" icon in the dock (bar at the bottom of the screen displaying the program icons). Wait for the "System Preferences" menu to appear.
Locate and click on the purple "Network" icon in the "Internet and Network" section of the window (in the middle of the window, directly under the "Hardware" section). Wait for the "Network" window to appear.
Click on the "Ethernet" connection tab at the right of the window to select it.
Click on the "Advanced" tab at the bottom-right-hand corner of the screen to bring up an Ethernet information window.
Locate the "Ethernet" tab at the top-right corner of the Ethernet information window and click on it. Wait for the "Ethernet ID" window to appear.
Locate the 12-digit alphanumeric "Ethernet ID" number at the top of the window. This is your iMac's MAC address. You can highlight this number and copy it to your computer's desktop for easy access by selecting "Copy" from the "File" menu. Then select "Paste" and click the desktop.