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We have recently encountered a situation where a Windows 10 machine assigned itself a Local-Link address of 169.254.0.33.

According to our reading of RFC 3927, section 2.1, this is not allowed. Ranges 169.254.0.xx, and 169.254.255.xx are reserved for future use and not to be used in auto-assignment.

Interestingly, reading the introduction on section 2 of the same RFC, it claims that, at least at the time of writing, MS Windows "is compatible with the rules in this section".

Is this non-compliance known ? Or have we hit a bug in Windows 10 ? Or are we mis-interpreting something ?

(Additionally, we timidly tried to see if we could get it to assign a different address (by enabling/disabling interface,...), but it always assigned the same address, 169.254.0.33. Not sure whether this is another non-compliance, though)

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Raul Benet
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    Windows uses a hash of the MAC address to pick the IPv4 Link-Local address and has for a long time so not surprising the interface keeps getting the same address. – Brian Jun 09 '18 at 11:10
  • Just curious - what does it matter? – Ryan Ries Jun 10 '18 at 15:52
  • @RyanRies I don't know which part of the question you refer to, so I'll address both. Regarding the IPv4 range - just that our software implements different policies depending on whether the PC is on a link-local address or not. So far, we were checking the range as per the RFC, but , as it is often the case, reality doesn't always follow RFCs. And regarding the fact that the address was constant, we were just curious. Again, following the RFC, it should be produced by a pseudo-random number generator. – Raul Benet Jun 11 '18 at 09:04

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In practice Microsoft uses the full /16.

"By default, the alternate IPv4 address is in the range from 169.254.0.1 to 169.254.255.254 with a subnet mask of 255.255.0.0."

Configuring TCP/IP networking
https://msdn.microsoft.com/en-us/library/dd163570.aspx

Greg Askew
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