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Re: Draft on Globally Unique IPv6 Local Unicast Addresses



I actually see a lot of value in the /56 proposal;  I really like the 
simplicity of creating the /56 from any MAC-48 in the network.  It 
accomplishes the uniqueness property without requiring central 
registration, and should serve organizations up to considerable size very 
well.  And it readily discourages the notion of "make up a prefix" for 
temporary or (temporarily) disconnected networks.

In the applications I can envision, a /56 should work fine, and an 
organization can "make more" based on additional MAC-48s.

The draft might contain some language like "An organization using this 
method for creating unique network prefixes SHOULD (or MUST?) retain 
physical custody of the network device contain the MAC-48 used to define 
the prefix, for as long the prefix is in use".

The way globally unique /48s based on a central registry are really only 
needed for very large organizations.

--On Tuesday, May 27, 2003 20:38 +0100 Zefram <zefram@fysh.org> wrote:

> Bob Hinden wrote:
>> There is a clear tradeoff between a longer ID (to allow for better
>> random  numbers or MAC addresses) and the size of the subnet field.
>>
>> Before revising the draft, I would prefer to hear from more people on
>> these  tradeoffs.
>
> Although I was one of those that suggested a technique that would generate
> /56 prefixes, I see great value in arranging for /48 prefixes where
> possible, for uniformity with RFC3177.  For randomly-generated addresses
> (both centrally allocated and individually allocated), an 8-bit format
> prefix and 40 bits of randomness seems like a good tradeoff.  40 bits
> seems to be about the amount of entropy we're aiming at, and a handful
> of bits either way makes no difference.  An 8-bit format prefix is not
> too wasteful of address space.
>
> The case for which I suggested /56 prefixes was generating a prefix
> from a MAC-48.  In that case, with 46 effective bits of identifier
> to fit into the prefix, we clearly can't get a /48.  It seems like a
> valuable technique, but would have to be an exception to the /48 rule.
> Of course, any site needing more than eight bits of subnet ID is likely
> to have many MAC-48s to play with if they have any at all.
>
> -zefram
> --
> Andrew Main (Zefram) <zefram@fysh.org>
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Hans Kruse, Associate Professor
J. Warren McClure School of Communication Systems Management
Adjunct Associate Professor of Electrical Engineering and Computer Science
Ohio University, Athens, OH, 45701
740-593-4891 voice, 740-593-4889 fax
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