We started with fec0::/10 for site-local addresses -- but that is now deprecated. Instead, we now have fc00::/7 reserved, with fd00::/8 dedicated to /48 nets for Unique Local Addresses.

What's the best practice for picking the least-sig 40 bits to give you an org-level /48?

...is everyone using the [SIXXS](http://www.sixxs.net/tools/grh/ula/) voluntary registry? (seems dubious to me)  

...just use [RFC4193](https://www.rfc-editor.org/rfc/rfc4193)'s guidance for picking (via it's algo)?  

...anyone have an online implementation of that algo?

Picking our 40 bits doesn't seem hard; I'm just trying to pick them the "right" way, if I can.