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 voluntary registry? (seems dubious to me)
...just use 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.

3 Answers 3


Its always best to stick to the relevant RFC when possible. In this case, your two options aren't mutually exclusive. SIXXS' generator should actually generate you a prefix based on RFC4193's algorithm.

So you would just generate your prefix using SIXXS' tool (or another one you find), and if you feel like it, register it with SIXXS.


The odds of someone picking the same range and then merging or making business with you should be very slim. As you mentioned SIXXS has a registration, not sure how much it is used and since it's really no authority there's no guarantee that someone wouldn't have picked the same range. I guess it's better than nothing though.

These are the tools I could find:


  • And here is a bash script that can be run locally.
    – hlovdal
    Commented Dec 24, 2020 at 23:49

If you do what the RFC says, the chance of an actual collision is very low (even though the birthday paradox applies), and since ULA addresses are only used internally on your own site, a collision doesn't matter in practice unless you merge your network with another one that, amazingly, is using the same ULA prefix.

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