I bought a second hand laptop from a guy called Marcus. It had Win 10 on it but I immediately installed a new SSD drive and loaded Linux Mint. I was surprised when I visited a cafe to find that it connected to The Cloud WiFi with the message 'Welcome back Marcus'. How is it recognising this laptop?
Because of the MAC address.
The MAC address is stored on your internet card and is shared with the router to route the internet packets. Since, you connected with the cafe wifi, it gave your MAC address to the cafe router which they then used to identify your computer using the information they previously stored.