Continuing the discussion from Device Identification / MAC Address:
Using a MAC Address would be a security vulnerability mainly because the MAC address uniquely identifies the running computer. Therefor it would be better to use the IP Address of the device instead as it is also a unique identifier and is not a security threat.
So I went onto my laptop and made a sample app that stores the users IP Address along with their name. The app consists of an API and a Realtime database.
NOTE: This app does not work without a Wi-Fi connection and will not work for a period of time upon being opened many times. This is because of the API’s request limit.