Mac addresses are a “unique identifier assigned to network interfaces for communications at the data link layer of a network segment”. They are similar to IP addresses on a local network, but are never changed. It is comprised of 12 letters/numbers that are hard coded into the network interface card, or NIC. Using software however, it is possible to give the NIC a temporary address to emulate. This can trick the network you are on to treat your computer as a different device.
Mac addresses have many uses, such as filtering on a private network. The network will only allow mac addresses on predefined list to connect. Another use would be to track a device’s movement when it passes by a Wi-Fi network. Many airports and public Wi-Fi networks also offer promos such as 30 minutes of free Wi-Fi. They track time based on your mac address, and blacklist it after you hit the time limit. By spoofing, you can reuse the promo multiple times, theoretically giving you unlimited wifi time.
Changing your mac address also has various uses, such as for privacy and to bypass any restrictions such as limited Wi-Fi time. This is often known by the term “spoofing” and can also be used for malicious purposes to steal packets and information on networks.
To change your mac address on Windows, you need to check your Network Adapter Properties and enter your desired address.
For OS X, you need to use the Terminal and enter a command, replacing en0 with your interface of choice and the mac being the one you want to spoof to.
On Android, you need root installed to change the address, which is different for every phone. An app like Macchanger can use root and BusyBox to change it for you.
By randomizing your mac address, you can avoid tracking on public wifi networks as well as trick them into believing you are a different device. You should always use a VPN in conjunction with random mac address, so no one can intercept your sensitive data.