Yesterday I applied for a HE IPv6 tunnel, and got one. The default tunnel consists of an IPv6 address, ending on ::2, and a routed /64 subnet. What struck me is that the address is in a different /48 subnet than the routed /64 subnet. That means that a whole /48 subnet is burned just to provide my router an IPv6 address.
As my former Sixxs IPv6 address and subnet had the same peculiarity, I guess there is a technical reason for that. Which one?
Not quite right.
You got A:B:C:D::2/64 for your router, and A:B:E:D::/64 as your subnet.
This means there is a /64 for your router to their router communication, and a /64 for your use. And then, presumably, each tunnel through that HE node gets is own unique D value, and shares the A:B:C:: and A:B:E:: subnets.
Admittedly, it might have been a better use of resources to give out A:B:C:0:0:0:D:2/112 for the router and A:B:C:D::/64 for the subnet, but it's probably better just not to try to explain using /112 (or even /126 or /127) subnets.