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I am trying to understand the prefixpolicies. Could somebody explain to me what the table from netsh interface ipv6 show prefixpolicies means on windows?

Precedence  Label  Prefix
----------  -----  --------------------------------
        50      0  ::1/128
        40      1  ::/0
        35      4  ::ffff:0:0/96
        30      2  2002::/16
         5      5  2001::/32
         3     13  fc00::/7
         1     11  fec0::/10
         1     12  3ffe::/16
         1      3  ::/96

Above is an example from my laptop. I know that the leftmost column means priority. The higher it is, the more likely that it will be used. I guess the label is just for identifying the different rules. But why are these specific rules there? I understand that the rule labeled 3 is representing the range the ipv4 addresses cover and I assume that this is rated lowest because then, windows will prefer any available ipv6 connection over its ipv4 counterpart. And the rule labeled 0 is probably the loopback address.

What are the other rules there for? Why do the addresses starting with 2002 and 2001:0000 have less priority than basically every other address? (Comparing the priority of rules 1, 2, 5)

Edit: ::ffff:0:0/96 regards the IPv4-mapped IPv6 addresses as described here

IPv4 address embedding is used to create a relationship between an IPv4 address and an IPv6 address to aid in the transition from IPv4 to IPv6. One type, the IPv4-compatible IPv6 address, is used for devices that are compatible with both IPv4 and IPv6; it begins with 96 zero bits. The other, the IPv4-mapped address, is used for mapping IPv4 devices that are not compatible with IPv6 into the IPv6 address space; it begins with 80 zeroes followed by 16 ones.

So that means my windows prefers mapped IPv4 connections over the real IPv4 addresses. Not sure why, just adding this for completeness.

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  • (eventually, I'm trying to find out how to temporarily enforce using IPv4 because my VPN does not support ipv6, but this is a different question)
    – lucidbrot
    May 30, 2017 at 7:02

1 Answer 1

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2002::/16 is a specific prefix used for 6to4 translations. It can be used to tunnel IPv6 through IPv4.

2001::/32 is reserved for Teredo, another IPv6 tunneling mechanism.

Both of these should have a low priority, since they should only be used if no native IPv6 is available.

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  • Thanks! Do you happen to also know why ULA (fc00::/7) has low precedence and why the afaik deprecated fec0::/10 is still listed? I assume 6bone (3ffe::/16) has the same reason as the two you mentioned, but according to wikipedia, this address space was returned to IANA so why is it there in my table?
    – lucidbrot
    May 30, 2017 at 7:48
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    For the ULA space it shouldn't matter much. You usually have ULA space with a more specific netmask (if any at all) so that route should always win, there's not much chance of overlapping routes. I have no idea why fec0::/10 is assigned to your interface or why 6BONE space is used, but I'm no Windows expert.
    – Teun Vink
    May 30, 2017 at 7:56

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