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my router (AVM Fritz Box) is supporting 6to4. So far it works but I want to understand what's going on. The last few hours I spent on a IPv6 book. There are many things I understand now but there is this one thing I can't figure out:

Router's addresses:
Public IPv4 address: (hex: 5dd1:5379)

Global Prefix:                   2002:5dd1:5379:00|00 :0000:0000:0000:0000/56
Global IPv6 address on WAN side: 2002:5dd1:5379:80 00|:0224:feff:fe19:0fde/64
Global IPv6 address on LAN side: 2002:5dd1:5379:00 00|:0224:feff:fe19:0fdc/64

I put a vertical bar (|) to the positition where the prefix ends.

So the Global prefix ends with :0000: but on the WAN side it ends with :8000:
As I understand it, this :8000: adress won't fit in the /56 prefix because the first byte is different from :0000:

What I expected was that the /64 subnets start in the second byte of this block.
E.g. :0001:, :0002:, ... :00FF:

But I am able to ping the global IPv6 address. Could someone explain to me what I got wrong?

share|improve this question
Your prefix is actually /48, not /56. – Michael Hampton Jan 17 '13 at 0:27
Thanks, Sander Steffann mentioned this too. I copied the addresses straight from the WebInterface. My FritzBox comletely auto configured this tunnel. – Reini Jan 17 '13 at 9:07
up vote 3 down vote accepted

Since you're using 6to4 you will have a whole /48 available to you. The structure of the address is as follows (probably including more detail than necessary for you, but just to be on the safe side):

  • IPv6 addresses are in hexadecimal
  • 2002 is the fixed prefix for 6to4 (16 bits)
  • 5dd1:5379 is your IPv4 address written in hexadecimal (2 * 16 bits)
  • Everything after that is yours (or your router's) to do with as you please :-)

You write this down as 2002:5dd1:5379::/48, which is technically the same as 2002:5dd1:5379:0000:0000:0000:0000:0000/48 if you write all the blocks and leading zeroes.

The /48 means that the first 48 bits are fixed (the 2002:5dd1:5379: part) and that everything else (0000:0000:0000:0000:0000 to ffff:ffff:ffff:ffff:ffff) is flexible (yours to work with).

With IPv6 every network (LAN) gets a /64. The first one is 2002:5dd1:5379:0000::/64 (which is usually shortened to (2002:5dd1:5379::/64) because :: means 'as many blocks of 0000 as necessary here). The second one is 2002:5dd1:5379:0001::/64, etc. Up to 2002:5dd1:5379:ffff::/64.

Yes, this means you get enough addresses for 65536 LANs!

The /56 mentioned as Global Prefix is just wrong. As I mentioned earlier it's actually a /48. The Fritz!box does seem to use the right addresses though despite showing the wrong prefix size. It uses ...:0000:... for its LAN interface and ...:8000:... for its WAN interface. Nothing wrong with those choices, just a display error it seems :-)

One important thing though: 6to4 is not always very reliable. Your router will send outbound IPv6 traffic over an IPv6-in-IPv4 tunnel to There are many routers on the internet with that address (a technique called anycast) and your outbound IPv6 traffic will end up using the one that the network thinks is closest. The same goes for the return traffic to your network. There are also many relays for that and which one is used depends on which one is closest to the other side. If either of those relays performs badly, is broken, not well maintained, doesn't have enough bandwidth etc etc etc then your IPv6 connection will be broken. And because the incoming traffic will use different relays depending on which other systems you communicate with debugging it is practically impossible.

I.e. Hurricane Electric ( / offers free IPv6-in-IPv4 tunnels that are statically configured. You will always use the same relay for inbound and outbound traffic and you'll know who is responsible for it if it fails. That will probably save you many headaches :-)

All of this is independent of your ISP. They don't have to provide any service related to 6to4 for this to work, although if you're lucky they provide an outbound relay for 6to4.

share|improve this answer
Thank You! And especially thank you for this sentence: "The /56 mentioned as Global Prefix is just wrong. As I mentioned earlier it's actually a /48." With this it all makes sense... – Reini Jan 17 '13 at 9:04

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