MAC stands for Media Access Control, and a MAC address is a unique identifier assigned to a network interface controller (NIC). A NIC can be a part of an Ethernet adapter, Wi-Fi or Bluetooth module used to connect to a network. Basically, this is the address of the network adapter hardware.

A typical MAC address contains six groups of two hexadecimal digits, separated by hyphens, colons, or without a separator:

00:0a:95:9d:68:16

Without a MAC addresses it would be impossible to distinguish between different network hardware adapters using the same IP address.

Is the MAC address accessible on the Internet?

MAC address is an internal address within a private network segment used for network communication between different devices. It is usually not visible or accessible outside of its surrounding network segment. Basically, a MAC address of a remote node on a global network is not available to another node in the global network.