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IPv4 vs IPv6 priority in Windows

Short version

Long Version

RFC6724 defined a change in how addresses should be preferred. With this change IPv6 is no longer the preferred address in nearly every case :(

This question, which was asked in June of 2012 was “fixed” by an RFC from September 2012. Depending on your Windows version, you either had this new policy out of the box (Windows 8.1), or likely already delivered through an update (Windows 8, Windows 7, Windows Vista).

We’re here because we want to use IPv6; we want that change undone.

How to put it back

If you get multiple IP addresses for a single host, your machine has to decide which address it will use. An example ranking might be:

  • IPv6 loopback
  • Native IPv6
  • Unique-Local addresses (ULAs), e.g. fdxx::
  • Site-local, e.g. fec0
  • 6bone
  • 6to4
  • IPv4compat
  • IPv4
  • Teredo, e.g. 2001

On your Windows machine, this ranking is called the prefix policy.

Prefix policy

You can view your computer’s prefix policy by running:

In the olden times (originally defined by RFC 3484), the prefix policy was:

So you see it would pretty much always use IPv6 (yay!):

  1. IPv6 loopback
  2. Native IPv6, ULAs, site-local, 6one
  3. 6to4
  4. IPv4compat
  5. IPv4
  6. Teredo

If you went through the effort to deploy IPv6: it just worked.

New Prefix Policy

In 2012 a new preference order was defined by RFC6724. Nowadays the prefix policy pretty much ensures that you’ll never use IPv6:

You’ll see that you will never be able to use your Unique Local Addresses, or site-local address; it’s perpetually broken:

  1. IPv6 loopback
  2. Native IPv6
  3. IPv4
  4. 6to4
  5. Teredo
  6. ULAs
  7. site-local
  8. 6bone
  9. IPv6compat

How to fix it?

What we want is to fix IPv6 so that ULAs are preferred over IPv4. At the very least we want to push the use of ULAs (fc00::/7) above that of IPv4:

Which is done by:

That will only keep it active until the next reboot. To make the change permanant:

If i:

  • went through the effort to generate a ULA global prefix for my /48
  • and choose a subnet id for my /64
  • and deploy ULAs to every machine in the enterprise
  • and update the DNS servers to return IPv6 ULA addresses in addition to IPv4 addresses

the least the computer could do is have the common courtesy to use the address.

Bonus Chatter

The fc00::/7 range is divided into two parts:

  • fd00::/8 – GlobalID prefix generated locally
  • fc00::/8 – ???

Nobody ever really decided was fc would be good for, and so just sits there.

The fd addresses are defined as:

fd [40-bit random GlobalID] [16-bit subnet] [64-bits for host assignment]

So if you generated a4d7f6dd66 as your cryptoghpcallly random 40-bit GlobalID, that gives you your /48:

  • fda4:d7f5:dd66:: /48
  • fda4:d7f5:dd66:face:: /64 (in the face subnet)
  • fda4:d7f5:dd66:face::825 as a host IP address

SixXS maintained a public database of Unique Local Address GlobalID prefixes in order to reduce the chance of collissions, e.g.:

  • fdee:e004:2208::/48: Apple Inc – Leopard OSX
  • fdd4:43c8:ba34::/48: TekSavvy – Danny Murray
  • fdac:afbd:fea1::/48: IBM Rational Build Forge – Chris Fuller

But due to slowing use, and the dubious value in the first place, SixXS discontinued the service in 2018.

Solution #1: Add a prefix policy to prefer IPv4 addresses over IPv6

Prefix policy table is similar a routing table, it determines which IP addresses are preferred when making a connection. Note that higher precedence in prefix policies is represented by a higher “precedence” value, exactly opposite to routing table “cost” value.

Default Windows prefix policy table:

Note that IPv6 addresses (::/0) are preferred over IPv4 addresses (::/96, ::ffff:0:0/96).

We can create a policy that will make Hurricane Electric IPv6 tunnel less favorable than any IPv4 address:

2001:470::/32 is Hurricane Electric’s prefix, 3 is a Precedence (very low) and 6 is a Label.

I could have used a more generic prefix, but I wanted to make sure that if and when I get direct IPv6 connectivity from an ISP, it will take precedence over IPv4.

If you adapt this solution, you need to substitute an appropriate IPv6 prefix instead of my Hurricane Electric one.

Solution #2: Tweak registry to make Windows always prefer IPv4 over IPv6

This solution is more generic, but more invasive and less standards-compliant. In the end, Windows will still modify the prefix policy table for you.

1)Open RegEdit, navigate to

2)Create DisabledComponents DWORD registry value, set its value to 20 (Hexadecimal).

for more info about this registry key, especially if DisabledComponents already exists on your system.


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