How do I set up IPv6 on my Airport Extreme?

Solution 1:

1. In the "Configure IPv6" menu, select "Automatic".
2. In the "IPv6 Mode" menu, choose "Native".

That should be all that's needed. The device should be able to autodetect the rest of its settings.

If that doesn't work, then you may need to check the settings with your ISP.

Source / more info: http://support.apple.com/kb/PH5098

Solution 2:

I know this is an old question, but I came across it looking for the same information - especially what the 'Enable IPv6 Connection Sharing' option did. So, I decided to update this thread on that information:

A reddit post from ~2016 asks what the 'Enable IPv6 Connection Sharing' is for. The answer there (from an Apple Certified-marked responder) says:

It allows IPv6 communication alongside IPv4...

I used an online IPv6 test pointed to by my ISP. With 'Enable IPv6 Connection Sharing' selected, I was able to pass the tests. When that box was unchecked, the test said that I didn't have an IPv6 address at all.

Note: I had the other two options selected as advised by @bennettp's answer. This gave me an IPv6 address and DNS servers from my ISP.