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Most websites configure their domain name server records only with A records for the use in IPv4 networks only. Some websites have configured AAAA records as well for use in IPv6 networks, using both IPv4 and IPv6.

This basically means that a server that only has an IPv6 address is not able to reach the IPv4 address and visa versa.

Does this mean that having an IPv4 address only is an availability risk? As well as only having an IPv6 addresses?

  • Given that many servers are still IPv4-only, most clients are either IPv4-only or dual-stacked. Hence, I think there’s very little availability risk having an IPv4-only server, while it is huge with an IPv6-only server. – user2233709 Apr 12 '17 at 11:50
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As always: it depends :)

Because so much of the internet is still IPv4-only there is a kind of implied responsibility to have IPv4 connectivity yourself. There are however advantages to having both.

The world has a huge shortage of IPv4 addresses, which means that there are a lot of hacks like carrier grade NAT, NAT64 and DS-Lite going on to keep users connected to the IPv4 internet. That means that when your service is only available over IPv4 then your users might be forced to use those hacks. When the service is available over both IPv4 and IPv6 the user can just use IPv6 and get better connectivity to your service.

This is probably also the reason that big services like Facebook and LinkedIn see 15% to 25% better performance over IPv6: those users don't need to go through a central NAT box at the ISP and don't lose performance there.

And if those central NAT boxes get overloaded (they shouldn't, if the ISP invested in enough capacity, but...) then IPv6 users won't even notice.

So I'd argue that there is already a certain availability advantage to deploying IPv6, and it will become a bigger advantage every month. In countries like Belgium almost 60% of the home users already has Ipv6 access.

  • 1
    One caveat I would add to this answer. I have frequently seen sites with a AAAA record pointing to a dysfunctional server. That is worse than having no AAAA record at all. In particular for users of NAT64 it is critically important that you only create the AAAA record once you can ensure reliable IPv6 connectivity to the server, as the existence of the AAAA record means those users can no longer fall back to the server's IPv4 address. – kasperd Jun 18 '17 at 7:42
  • Indeed. That's why we started NAT64Check.org – Sander Steffann Jun 18 '17 at 11:42
  • Nice. I'll try to remember to test that next time I am on a network using NAT64. – kasperd Jun 18 '17 at 18:59
  • It's to test how websites would work if the client device was behind NAT64, so you don't need to be behind NAT64 yourself :) The test will open the website three times: with IPv4-only, with IPv6-only and with IPv6+NAT64. It will make screenshots and compare them so people can check if their website still works. – Sander Steffann Jun 19 '17 at 6:46
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    I see. It does seem like one of your servers is having problems. The test just hangs forever. – kasperd Jun 19 '17 at 6:54

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