[simon_vetter]

iOS8 has been reported to be dual stacked (v6 + v4 addresses) on the cellular side of things on Verizon and a bunch of other (mostly European) networks.

What Apple did with iOS9 is improve its address selection algorithm to prefer ipv6 in a majority of cases (if both the device and the server are dual stacked and unless v6 path latency is much greater than its v4 counterpart, v6 will be used).

In contrast, ios8 would prefer v4 over v6 when both endpoints were dual stacked.

Also, from what I've seen, iOS devices won't provision ipv6 addresses on their cellular interface unless their Carrier Profile says they should, so you might have encountered devices not getting ipv6 addresses even though they were connected to a dual stack network. Wifi always uses v6 if the network supports it.