[Major cross post, set reply-to to NANOG, please honor it... ] [Note: I am not talking about ULA Central here, though it could apply] To stop the pesky emails about ULA, I hereby present a (partial) solution to this problem. We have ULA as per RFC4193. With a little math one can generate a ULA prefix sized /48 which is most likely globally unique. According to the RFC with 10.000 connections the collision probability is still a huge 4.54*10^-05. Some people have expressed a problem with this, especially when they want to use the generated ULA prefix for their large organization. The simple, cheap and quick solution: a ULA registry. This comes close to ULA-Central, but what we do is simply make a list of the "in use" ULA's, without any allocation policies whatsoever except: first come first serve and no cash to pay. The tool is here: http://www.sixxs.net/tools/grh/ula/ The page allows one to generate a ULA based on a given MAC address (multicast + locally defined + unregistered are not accepted) and then register it. The Name + Email are mandatory to restrict abuse a little bit. Email is not shown, except in whois and can be used to possible contact people who are leaking ULA prefixes. Note, it is indeed a _partial_ solution. When somebody still generates the same ULA prefix and does not check the list you can still collide. As this is primarily a problem for big organizations, they should be checking this list for collisions and registering their prefix when they see that they have a need for that. The list is available from the website, but also per whois.sixxs.net which is now serving up fd00::/8. It does not cover ULA-C (fc00::/8). As SixXS is a private hobbyproject kind of thing, we of course are not liable for anything that this might cause to you, your family, company etc. But enjoy it nevertheless. Full copies of the list in inet6num format are available from the site. Greets, Jeroen PS: GRH still classifies ULA's as "BAD" of course PPS: Thanks to Peter van Dijk for the multicast/locally defined MACs PPPS: Folks registering prefixes like crazy will nicely get locked out automatically, so don't even try... PPPPS: Thanks to SUZUKI Shinsuke and Holger Zuleger for the generator.