[aaronmdjones]

fd00::/8 (within fc00::/7) is still reserved for this purpose (site-local IPv6 addressing).

fc00::/8 (a network block for a registry of organisation-specific assignments for site-local use) is the idea that was abandoned.

Roughly speaking, the following are analogs:

169.254/16 -> fe80::/64 (within fe80::/10)

10/8, 172.16/12, 192.168/16 -> a randomly-generated network (within fd00::/8)

For example, a service I maintain that consists of several machines in a partial WireGuard mesh uses fda2:daf7:a7d4:c4fb::/64 for its peers. The recommendation is no larger than a /48, so a /64 is fine (and I only need the one network, anyway).

fc00::/7 is not globally routable.

[rerdavies]

So in my case, I guess I need to blame the unconfigurable cable router my ISP provided me with? Since there's no way to provide reservations for IPv6 addresses. :-/

[aaronmdjones]

Right. OpenWRT, for example, will automatically generate a random /48 within fd00::/8 to use as a ULA (unique local addressing) prefix for its LAN interfaces, and will advertise those prefixes to its clients. You can also manually configure a specific prefix instead.

e.g. Imagine the following OpenWRT setup:

ULA: fd9e:c023:bb5f::/48

(V)LAN 1: IPv6 assignment hint 1, suffix 1

(V)LAN 2: IPv6 assignment hint 2, suffix ffff

Clients on LAN 1 would be advertised the prefix fd9e:c023:bb5f:1::/64 and automatically configure addresses for themselves within it. The router itself would be reachable at fd9e:c023:bb5f:1::1.

Clients on LAN 2 would be advertised the prefix fd9e:c023:bb5f:2::/64 and automatically configure addresses for themselves within it. The router itself would be reachable at fd9e:c023:bb5f:2::ffff.

Clients on LAN 1 could communicate with clients on LAN 2 (firewall permitting) and vice versa by using these ULA addresses, without any IPv6 WAN connectivity or global-scope addresses.