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Depending on your system, this might be called Terminal, Xterm, Shell, Command Prompt, or something similar. It can usually be found in the Accessories folder in your Applications (or the equivalent). Type ifconfig -a and press Enter. If you are denied access, enter sudo ifconfig -a and enter your password when prompted. Scroll until you find your network connection (the primary Ethernet port is labeled eth0). Look for the HWaddr entry. This is your MAC address.
Open the terminal. Open the interface configuration. Find your MAC address.