It is important to know that your computer may have more than one network interface, each with its own hardware (MAC) address. Make sure to register the right one and not to mix up e.g. those for Ethernet (cable) and wireless connections.

Checking the labels

If you have got a separate network card (because there was no built-in device) then usually you will find its MAC address written on it, as shown in the example photos.

In this case the MAC address (for the wireless network!) would be 00095CA04ABA. Sometimes you will find it written in the form 00:09:5C:A0:4A:BA, or 00 09 5C A0 4A BA or 0:9:5c:a0:4a:ba. In every case it is a sequence of up to twelve hexadecimal digits, optionally separated by colons or spaces. A hexadecimal digit is either a normal digit (0-9) or a letter in the range A-F that may be written in upper or lower case.

Some manufacturers also attach labels to the laptops which indicate the MAC addresses of the built-in devices. Again, if there are both wireless and cable interfaces, pay attention to which is which.

Through the Operating System

Windows

Open a Command Prompt window (Start button, Programs, Accessories). Type ipconfig /all | more and look for the section about the wireless network interface. There you should see an entry about the hardware address, showing the requested number consisting of 12 hexadecimal digits.

Linux

In a Terminal window, run the command /sbin/ifconfig | more and look for the section regarding the wireless network. There you should find the MAC address under the title HWaddr. Again, be careful that you take the addresss for the wireless interface, not for your Ethernet card.

Mac OS X

Open the System Preferences, Network. In the Show: box, select AirPort. Now activate the right-most tab below (Ethernet). You will find the MAC address as first item, under the title Ethernet ID.