Physical Composition of a MAC address on a NIC card - what would it look like under a microscope?

Question

Here can I find a description of how a MAC address is physically stored on a NIC card? I'm not looking for an explanation of the engineering process that put it there. I'm looking for a description of its physical composition (i.e. bit values [microscopic up and down things] stored on the NIC chip making up 48 bits).

Answer 1

It's probably not possible to answer this completely as there is no required method a network interface must use to physically store it's built-in or "burned in" address. Furthermore, it may be different for each manufacturer or chipset of NIC.

However, taking the BCM57xx series NIC as an example, in the programmer's guide it mentions that the NIC registers are initially populated from "NVRAM", and that NVRAM in this case is flash or serial EEPROM.

An excerpt from page 88 of the above .PDF:

Broadcom NetXtreme controllers require the use of an external non-volatile memory (NVRAM) device (Flash or SEEPROM), which contains a bootcode program that the controller's on-chip CPU core loads and executes upon release from reset. This external NVRAM device also contains many configuration item s that direct the behavior of the controller, enable/disable various management and/or value-add firmware components, etc. All configuration settings are default-configured in the official release binary image files provided in Broadcom's CD software releases. However, the settings chosen as default by Broadcom may not be what best suits a particular OEM's application so may need to be changed by the OEM

You can search further for microscopic images of NAND cells if you want - but the actual bits are electrical charges and I don't believe they are visible. In the bit of reading I've done about it, flash memory is a form of an EEPROM so I believe serial EEPROMs would be in the same boat regarding that.

I would guess that most NICs these days operate the same way.

Answer 2

MAC addresses are 12-digit, 48 bits in length, hexadecimal numbers (64 bits is also in use). For example, this is a 48-bit MAC address:

00:A0:C9:14:C8:29

where the first six characters 00:A0:C9 is the Organisational Unique Identifier (OUI) of a manufacturer. In this case, Intel Corporation (See here for explanation of the first six characters and its meaning).

The other six characters (14C829) represents the serial number assigned to the hardware (in this case, a router) by the manufacturer.

I wouldn't trust wikipedia.org as an authoritative source, because half the time, it's written by people who understand only half of the whole fact, and editted by those who knew less than these halves. You would be better off visiting official standard organisations like Institute of Electrical and Electronics Engineers if you have a query.