A Media Access Control address (MAC address) is a unique identifier assigned to network interfaces for communications on the physical network segment. MAC addresses are used for numerous network technologies and most IEEE 802 network technologies including Ethernet. Logically, MAC addresses are used in the Media Access Control protocol sub-layer of the OSI reference model.
MAC addresses are most often assigned by the manufacturer of a network interface card (NIC) and are stored in its hardware, the card's read-only memory, or some other firmware mechanism. If assigned by the manufacturer, a MAC address usually encodes the manufacturer's registered identification number and may be referred to as the burned-in address. It may also be known as an Ethernet hardware address (EHA), hardware address or physical address.
MAC addresses are formed according to the rules of one of three numbering name spaces managed by the Institute of Electrical and Electronics Engineers (IEEE): MAC-48, EUI-48, and EUI-64. The IEEE claims trademarks on the names EUI-48 and EUI-64, in which EUI is an acronym for Extended Unique Identifier.
HFC = Hybrid Fibre-Coax. This abbreviation refers to the port of the cable modem that connects to the CATV coax cable. Some cable ISPs require you to register with them the HFC MAC address of your cable modem.
The HFC MAC address of the cable modem is stamped on the manufacturer's label on the back or underside of the cable modem, or on the label of the box in which the cable modem was sold.
Cable modems might have multiple MAC addresses, one for the HFC side (cable TV coax socket), and one for the CPE side (Customer Premises Equipment, the ethernet socket). USB-capable cable modems also have two further MAC addresses: one for the cable modem USB socket, and another for the emulated network interface in the USB driver in the PC. Although it is possible to discover the CPE MAC address(es) of the cable modem, there is no application or procedure that requires them.