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 <[EMAIL PROTECTED]> 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) <[EMAIL PROTECTED]> -------------------------------------------------------------------- IETF IPng Working Group Mailing List IPng Home Page: http://playground.sun.com/ipng FTP archive: ftp://playground.sun.com/pub/ipng Direct all administrative requests to [EMAIL PROTECTED] --------------------------------------------------------------------
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