Spoofing

https://de.wikipedia.org/wiki/Spoofing

Every device has a MAC (Media Access Control) address. It's like a serial number of the hardware. The MAC address is a twelve-character hexadecimal number. The first three pairs represent the company that made the network interface (e.g. Intel). The last six numbers are a unique address.

a4:c3:f0:85:ac:2d

However, an interesting thing with MAC addresses is that they can be faked or "spoofed" in a process known as spoofing.

This spoofing occurs when a networked device pretends to identify as another using its MAC address. When this occurs, it can often break poorly implemented security designs that assume that devices talking on a network are trustworthy. Take the following scenario: A firewall is configured to allow any communication going to and from the MAC address of the administrator.

If a device were to pretend or "spoof" this MAC address, the firewall would now think that it is receiving communication from the administrator when it isn't.