EUI-64 IPv6 addresses are generated from MAC addresses by taking the network prefix (64 bits) and appending the first half of the MAC address, then FF:FE, then the second half of the MAC. Is there a reason for the FF:FE? Other than 64+64=128.

Note: I didn't mention in the above that some bits in the prefix, specifically the "global" identifier and the "unique" identifier, get flipped.

1 Answer 1


0xFFFE was chosen by the IEEE as a reserved value to reflect the fact that a given address is in the modified EUI-64 format.

It's important to note that the first and second half of a MAC address actually have significance - the first being the organizationally unique identifier (OUI) that identifies the NIC manufacturer/class and the second being a number assigned by said manufacturer.

So - we have [8-byte prefix]+[3-byte OUI]+[FF-FE, 2 bytes]+[3-byte assigned NIC value] = 16 bytes, 128 bits.

Here is a good reference explaining the above.

  • Don't forget to flip the U/L bit.
    – Ron Maupin
    Dec 6, 2016 at 14:31

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