Well -that's not entirely true. RFC4193/ULA has a Local Bit that, in my experience, is almost set to 1, so the prefix is FDxx.... At a prior company we roll

Well -that's not entirely true. RFC4193/ULA has a Local Bit that, in my experience, is almost set to 1, so the prefix is FDxx.... At a prior company we rolled out about 25 million nodes all in RFC4193 space (non-routability was a design objective) - and, just to be within spec, we did indeed grab a random /48 for each customer instance we rolled out (and made very good use of the 65K networks we had available within that /48). But - what if that Local bit is set to 0 - then: Set to 1 if the prefix is locally assigned. Set to 0 may be defined in the future. Presumably there was some thought that there might be a Global Registry that the OP was referring to? Concur with the rest- and, honestly, RFC 4193 is one of the simply written RFCs out there - understandable within 30 minute read - I'd encourage everyone to dig into it.