You might think that connecting anonymously to a public Wi-Fi network doesn’t reveal much about you. You might be using a VPN (virtual private network) to protect everything you do. But even if you aren’t, the vast majority of websites and email servers (and pretty much all those run by companies) use client-to-server encryption. But what if you could be tracked anyway?
Apple has a solution for this as it does for many tracking systems. The company’s trick lies in how Wi-Fi (and ethernet) adapters identify themselves over a local network when connecting wirelessly or via an ethernet cable. Every network adapter has a factory-assigned unique address baked in at the time of manufacture. It’s called a Media (or Medium) Access Control address; the abbreviation is MAC, confusingly enough, but it has nothing to do with Macintoshes.
Where an IP (Internet Protocol) address defines your machine’s location on the internet, a MAC address defines it on your local area network (LAN). The MAC is in part how devices on a LAN all communicate with one another, whether over Wi-Fi or ethernet.
Read more at Macworld.com