From: Computer Beginner's Guides
Terminal: CTRL+ALT+T keys simultaneously

Once the Termina launches, type this command and then hit the ENTER key:

ifconfig

The ifconfig command will generate an output with a great deal of information.

Fortunately, most of it is useful. The “eth0″ refers to the first Ethernet connection on your system. The “indet addr” displays your system’s IP address, while “Mask” shows the subnet mask. “HWaddr” shows your Ethernet adapter’s MAC (Media Access Control) address, which is (theoretically) unique to each adapter. (Some wireless networks require you to supply your MAC address before allowing your system to connect.)
 

To find the MAC address:

ifconfig | grep HWaddr