IPV6 planning

Owen DeLong owen at delong.com
Mon Mar 7 23:51:06 UTC 2016

> On Mar 6, 2016, at 17:57 , Baldur Norddahl <baldur.norddahl at gmail.com> wrote:
> Den 6. mar. 2016 13.41 skrev "Karl Auer" <kauer at biplane.com.au>:
>> Dunno about "harsh", but RFC 2464, section 4 says that the prefix must
>> be 64 bits. By (extremely strong) implication, a host must not use a
>> prefix of any other length to perform SLAAC. I say "extremely strong"
>> because the entire description of how an IPv6 Ethernet interface
>> identifier is formed depends on it being composed of the prefix plus an
>> EUI-64 identifier. Later RFCs firm up the requirement and apply it in
>> other contexts.
> But the most popular OS (Windows) completely ignores all of that and makes
> up an identifier not based on EUI-64. Everyone are happy anyway. The RFC
> should have let identifier selection as an implementation detail as the
> risk of collision is almost non existent given a sufficient random
> selection and we have duplicate address detection as a safeguard.

To the best of my knowledge, Windows actually generates three addresses…

1. Subnet Stable quasi-randomized address unrelated (or at least not reversable to) MAC address.
2. Privacy address which rotates frequently (for some definition of frequently).
3. Stable address related to MAC address.

The 3rd one is standard SLAAC.
The second one is standard privacy extensions.
THe first one is unique to Windows. You’ll get the same address every time you connect to the same subnet, but you won’t see that suffix for that host on any other subnet.


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