Real world use for the U/L bit?

Brian E Carpenter brian.e.carpenter at
Mon Nov 15 04:21:32 CET 2010

On 2010-11-15 14:17, George Bonser wrote:
>> -----Original Message-----
>> From: Brian E Carpenter [mailto:brian.e.carpenter at]
>> Sent: Sunday, November 14, 2010 4:10 PM
>> To: George Bonser
>> Cc: Roger Wiklund; ipv6-ops at
>> Subject: Re: Real world use for the U/L bit?
>> George,
>> The u/l bit is quite orthogonal to ULA prefixes.
> Yes, my apologies, I was for some reason thinking it was having to do with the reserved bit in the local prefixes. The half that isn't currently used and some have suggested it be used for a "global private" address space.

In fact that bit is reserved for hypothetical *registered* unique local address
prefixes - not supposed to be any different from pseudo-random ULA prefixes,
but meant for people frightened of the birthday paradox. Personally, I think
it's a bit silly.


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