Suddenly FD...-addresses, aka Unique Local IPv6 Unicast Addresses

Suddenly I saw IPv6 addresses starting with FD in my ifconfig.

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?

Update (2018-11-11):
Both Android and MacOSX show the FD... addresses too.