I also forgot the mention that MAC addresses can easily be spoofed with tools like macchanger that can be used to bypass simple NAC systems or MAC address filtering with relative ease. This is how I usually bypass captive portals for free wifi by changing my MAC address to one that's already authenticated to the WAP after searching for one with the aircrack-ng suite.
What kind of information is leaked through Wi-Fi?
Levi Hughes
Bentley Young