New ISP for me, apparently using ipv6, but I don't fully understand ...

www.myipaddress.com doesn't have any AAAA records (A records for IPv6 addresses) - it just isn't reachable via IPv6. That's most likely the case with your own server too.

Edit: The forum converts www. stuff to http:// links; I only wanted to write that as a hostname.

AAAA records being part of DNS, does that mean the "conversion" is happening at the client DNS or ISP level? It would have to be, right? Sorry if I'm using the wrong terminology. 

My dev server does it too. It's just a Ubuntu desktop machine. Maybe when I have some free time I can figure out how to accept IPv6.

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