What is a NIC address?

Windows and Mac version:

The NIC address is a unique code embedded into your network device at the time of manufacture. It consists of a six-byte code that has more than 281 trillion possible variations. Traditionally only used for network interface cards, today's computers generally have several NIC addresses to help to identify many other parts of your system, including your Ethernet card, Bluetooth address, SCSI address, etc.

iOS version:

Apple uses a proprietory method to identify eligible devices which may or may not include the NIC address.