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My PCs MAC address ends with FF:FF:FF. I am little confused about this address. As per my knowledge MAC address is of 12 Hex digits (first 6 correspond to Company and second half to a unique NIC). And as NIC numbers may be between 00:00:00 and FF:FF:FF (both exclusive).

00:00:00 and FF:FF:FF are not assigned.

Is this a manufacturing mistake?

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Are you sure that this address isn't changed in your network card preferences? – week Nov 15 '12 at 15:20
How it can be changed? Isn't that a fix Address. – shekhar Nov 16 '12 at 6:48
Not necessarily, it is a fix address and you can't rewrite that directly in interface, but you may override it by system settings. If you are using windows that it is in "Preferences -> Configuration" of that network device. In Linux you can change it by command ifconfig eth0 hw ether aa:bb:cc:dd:ee:ff. – week Nov 16 '12 at 10:39

It is true that all 0's or all F's have special group meanings, but that includes the vendor ID, so a FF-FF-FF in the OUI is fine as long as the vendorID portion is not FF-FF-FF as well.

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