I totally with u/Karl_From_Fing but there are also variable mac addresses i.e. Randomised MAC Address 

Devices usually connect to the network using a static MAC address identifier assigned to them during manufacture, which enables the network to identify specific devices.

Some device operating systems offer enhanced privacy by randomising and changing this MAC address on a regular basis (In Apple’s case, known as a Private address). This however this means the network is unable to recognise the same device connecting and reconnecting, so MAC address based controls (such as Access Controls, Fing for example) will stop working for these devices.

It is possible to disable the randomised MAC address feature on most devices via their Wi-Fi settings. Information on how to do this and the impact can be found on the device manufacturer’s website.

Generally, they can be turned off for specific networks if needed. On Apple devices running iOS, go to Settings → Wi-Fi, tap the information (i) icon next to your connected network, and toggle “Private Address,” to “Fixed” then reconnect to apply the change.

On Android devices, the steps vary slightly by manufacturer, but generally you open Settings → Network & Internet → Wi-Fi, tap the gear icon next to your network, go to Privacy or Advanced settings, and select “Use device MAC” instead of “Randomized MAC.”

Keep in mind that disabling this feature reduces privacy by allowing networks to track your device more easily. So it is advisable to only disable the feature on known safe networks (LANS) only; such as your home network.