Re: "IPv6 was designed to run alongside IPv4 from the beginning. "
So you got AFAIK, three different ways to have addresses assigned and have to ensure they are configured correctly
Indeed. So, my ISP enables IPv6 for me (at my request), which means that my router now has an IPv6 address. My firewall, that sits behind the router, also gets an IPv6 address.
Unfortunately, one that my router doesn't seem to understand, despite the fact that it's in (theoretically) the same prefix. So my devices inside the firewall can get their IPv6 addresses from the firewall and can see the firewall and all the IPv6-enabled devices inside. But nothing past the external interface of the firewall. The external interface of the firewall can see the internal interface of the router, but not the outside interface and nothing at all outside. So even manually looking up the IPv6 address of an external site and trying to ping6 it gets me a total failure to ping (IPv4 ping can reach it fine).
My firewall rules are OK (anything IPv6 can see anything outside on IPv6 - I'll get it working first then worry about protecting things). My router isn't running any firewall.
Can I work out how to fix it? No. Lots of theoretical stuff on "this is how it should work" that is totally unhelpful. And, after day of trying to make it work, several bottles of gin are starting to look mighty attractive, even though my head is already hurting trying to work my way through the broken IPv6 model..
In the end I gave up and turned off IPv6. Nothing I had needed it anyway..