I was having lots of fun (read no fun at all) over the past couple of days attempting to progress through the Hurricane Electric IPv6 certification. One of the tests involves the HE server retrieving a file via IPv6, which I was repeatedly failing. Given that I could access the file via IPv6, I was struggling to understand why he.net couldn't.
It turns out that the Fritz!Box has an IPv6 firewall, which prevents inbound connections to local hosts. (on the fritz!box, it's Internet -> Permit Access -> IPv6). There's a box to put a description in, which I've used as the hostname, then a somewhat cryptic four boxes labelled up as "Interface ID". After a bit of prodding and guesswork, I decided that this was the final four groups of hex digits in the IPv6 address. A bit more fettling, and a couple of "delete it and start again"s, and lo, there was
light external IPv6 access to my local servers.