MAC addresses
Every network adapter has what's called a Media Access Control address (usually shortened to MAC address). It's a six-byte identifying number permanently embedded in the firmware of the adapter, and is readable by the network and the operating system of the device on which the adapter is installed. All modems have a MAC address; so do all Ethernet cards. The address must follow the standards set by the Institute of Electrical and Electronics Engineers (IEEE), which sets computer networking standards. Basically, the address is a six-pair set of hexadecimal numbers, for example, a1-c2-e3-44-5f-6d.
The purpose of the MAC address is to uniquely identify every node on a network; nodes include workstations and devices like printers. Every adapter has a unique MAC address. Even two identical models from the same manufacturer will have distinct MAC addresses.
Note: Some literature calls the MAC address the physical address, the hardware address, or the adapter address for various reasons. While that's not wrong or inaccurate, the proper way to refer to that address is to call it the MAC address.
