On Linux 3.13:
inet6 addr: fdd6:5a2d:3f20:0:213:77ff:fefa:63e3/64 Scope:Global
inet6 addr: fdd6:5a2d:3f20:0:6001:f53f:1e8:3850/64 Scope:Global
and on Linux 4.14:
inet6 fdd6:5a2d:3f20:0:ce90:a2cc:655b:416e prefixlen 64 scopeid 0x0
It appears these are "Unique Local IPv6 Unicast Addresses", defined in RFC4193. They are world wide unique (well, at least: very probably unique), and to be used within a site.
At first I thought this was a new Linux setting, but these addresses also showed up on my old Ubuntu 14.04.
So ... what is going on? Ah, it's my new Fiber modem/router Genexis Platinum that is providing these addresses.
My use so far: you can use the FD address without specifying the interface. So handier than the FE80 addresses.
I checked a Windows 10 machine, but Windows 10 did not show these addresses. Maybe a Windows policy?
Both Android and MacOSX show the FD... addresses too.