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What "World IPv6 Day" Means To Sysadmins

I'm going to be extremely technical here. Sysadmins should really understand what World IPv6 Day is.

Does it mean the world is converting to IPv6 today? No. No, not at all. The upgrade requires technical planning and work. It can't happen without your help and without your knowing. Besides, the plan is to move to "dual stacked" IPv4+IPv6 on all hosts/networks and run that way for a good long time.

Does it mean my ISP is going to enable IPv6 on my connection? No. Not at all. (Seriously, folks, when was the last time your ISP added a feature without you having to beg for it first?)

So what does it mean?

You know that an IPv4 host as a DNS "A" record. Right?

You may know that an IPv6 host has a DNS "AAAA" record. (it is 4 times longer than IPv4, get it?)

Do a DNS lookup of and you'll get both an A record and a AAAA record.

A machine that is "IPv4-only" will ignore the AAAA record.

A machine that is "IPv6-only" will ignore the A record.

A machine that is "IPv4 and IPv6 dual stack" will try AAAA first.

Makes sense, right?

Macs, Windows, Linux boxes and a lot of other equipment comes with IPv6 enabled so that if you plug into a network that handles IPv6 it will just magically work. This is freakin' awesome.

It also causes a small problem.

Here's the problem. If you plug into a badly configured router, your machine might think there is IPv6. Maybe there is just for that subnet but not from that subnet to the ISP. What would a machine do in that situation? It would try the AAAA-record, and those packets would go nowhere. Eventually the machine will try the A-record, but that could be 30 seconds away. Not a happy experience.

The machines with this problem are about 0.05% of the internet. Not a lot, but not zero.

So what do sites do?

Google (for example) has AAAA-records for nearly all its services. However, if the DNS query comes over IPv4 it doesn't show you the AAAA-records. It only gives them to users that do their DNS queries over IPv6.

This is pretty cool if you think about it. If your DNS query could get to Google over IPv6, it must be safe for you to do your http over IPv6 too. Smart, right?

Google, Bing and tons of other sites do this trick.

Now that you know all that, I can tell you what "World IPv6 Day" is.

It is 24 hours where sites are going to give you the AAAA-record AND the A-record even if the DNS query came over IPv4.

That's it? Basically, yes, that's it.

How does this affect you?

You see, it isn't about the 0.05% of the users. It is about YOU and YOUR NETWORK which is causing one of those 0.05% to be in that 0.05%!

If you find a machine that is having problems, you can disable IPv6. However you should also fix whats wrong with that network. Disable IPv6 on the router or (much better) fix the IPv6 connectivity between the machine and "the internet".

Got it?


Here's what you can do:

Oh, and the day also raises awareness of the IPv6 issue in the media and in the technical world. That's a good thing too.

Thank you for listening.

P.S. If your machine has IPv6 enabled it will create a "link local" IP address. That isn't enough for it to think it has IPv6 connectivity.

P.P.S. People that attend LOPSA PICC had an amazing IPv6 half-day tutorial. Neener neener!

Posted by Tom Limoncelli in IPv6


14 hours in to World IPv6 Day and we haven't been able to find any of the mythical 0.05% of failing users. They should at least be complaining on Twitter, which is IPv4 only today...

This has been as anticlimactic as Y2k, if not more so. I hope a lot of sites just leave their IPv6 up.

I find lot of DNS hosts have not yet implemented the ability to add AAAA records meaning errors are throw when trying to add ipv6 to your zone

Also worth a mention: World IPv6 Day is June 8, 2011.


Umm.. that google IPv6 Test address lies. "You don’t have IPv6, but you shouldn’t have problems on websites that add IPv6 support."

I do have IPv6, but doesn't even have an AAAA record (though does).

Excellent question!

It doesn't have a AAAA record to make sure that everyone can access it properly. However, if you look at the HTML, you'll see code that has other addresses and does a proper test.


> I find lot of DNS hosts have not yet implemented
> the ability to add AAAA records meaning errors
> are throw when trying to add ipv6 to your zone

Good point. Today is a great day to write to them and complain!