In the OSI model, the third network layer is responsible for IP address and the second data link layer is responsible for MAC address. The MAC address is used to uniquely identify the network card in the network. If a device has one or more network cards, each network card needs and will have a unique MAC address.
Most of the ways to access the Internet are to organize the hosts together through the local area network, and then connect with the Internet through devices such as switches or routers. In this way, the problem of how to distinguish specific users and prevent IP addresses from being stolen appears.
Because the IP address is only a logical identification, anyone can modify it at will, so it cannot be used to specifically identify a user.