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The AVM FritzBox WiFi router has a feature to set the required MAC address from the configuration GUI.

When we change the MAC address, is this change done virtually at the firmware level, or does the change happen at the hardware level by changing the MAC stored in modem's EEPROM?

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migrated from Sep 14 '12 at 23:48

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What happens if you disconnect power and restart it? – stark Sep 14 '12 at 23:11
Belongs on Super User or (maybe) Server Fault. – millimoose Sep 14 '12 at 23:13
Also, a possibly helpful data point: on Tomato firmware the change is persistent. I believe the idea is that this is useful to spoof the MAC of the laptop used to set up the connection, since ISP provisioning systems track users by tying a MAC to your account. – millimoose Sep 14 '12 at 23:18
Yes, this option is to spoof MAC of the PC connected to modem when ISP expects a special MAC address. Just wanted to know do this affect EEPROM, how this realized. – fxgreen Sep 14 '12 at 23:44

Any network protocol requesting the MAC address of a host needs to take that host's word for it. No network protocol can really "reach into" remote hardware and verify the claim of a host replying to ARP or similar.

Consequently, there's no real need to provide the capability to modify whatever facility a NIC manufacturer uses to store or contain its MAC address. Many, if not all, NICs have microcontrollers and some element of firmware, and therefore RAM. Some wired NICs, but many wireless LAN cards especially depend on the host side driver to load firmware into the card.

It's sufficient to have this firmware copy the MAC from a micro-ROM or similar on boot to the microcontrollers internal RAM, and provide some type of interface (i.e. PCI registers, etc.) over the host side bus to change the in-RAM copy. There are many legitimate reasons to change a MAC address so manufactuers don't tend to leave it out, except some wireless NICs.

There may be some weird network hardware that has a flash ROM or similar that contains the MAC address. Some very old ISA NICs may have a DIP EEPROM on it - with old (and new, really) PC hardware anything is really possible.

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well, so this MAC address changes on WiFi router are done virtually at the firmware level? (not real change MAC in EEPROM) – fxgreen Sep 15 '12 at 0:35
Overwhelmingly likely it's at the firmware level, probably stored in a configuration file or other similar structure, accessible by the software running on the units CPU, in the same flash memory that the firmware itself is stored within. – LawrenceC Sep 15 '12 at 0:35

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