Re: what problem is solved by proscribing non-64 bit prefixes?

[Pekka Savola <pekkas at netcore.fi>]

On Tue, 30 Sep 2008, Brian Dickson wrote:
> Dunn, Jeffrey H. wrote:
>
> My basic question is: What basic engineering problem is solved by
> proscribing non-64 bit prefixes?

If the non-64 prefixes had not been proscribed, we wouldn't have been 
able to use the existing engineering method to develop CGA/SEND 
specifications.  These are being leveraged in SHIM6 and potentially in 
other applications as well. Likely we couldn't even solve these 
engineering problems (at least without major drawbacks/other 
assumptions) if we couldn't have made assumptions about the 
widely-used prefix length.

That said, I personally use non-64 bit prefixes on point-to-point 
links between routers, but I have done so willingly and knowing that 
if the IETF develops anything fancy new stuff, I might not be able to 
use it or I might need to renumber.

> When managing such a scheme alongside an IPv6 prefix which needs to 
> be assigned to the same set of servers, which are all dual-stack, 
> the *number* of prefixes, their *relative* numbering, and the host 
> *addresses* within the prefixes, it is quickly apparent that use of 
> only /64 prefixes makes for a management nightmare, particularly if 
> renumbering of prefixes and/or servers occurs, e.g. re-balancing the 
> VLSM arrangement itself in IPv4-land.

I don't understand why *relative* numbering (i.e: overlapping subnet 
masks) is important in IPv6, and I'm not sure if I see the case even 
for v4.  Could you enlighten me?

I assume you refer to a scenario where on the same broadcast domain 
there are hosts which are configured with say A.B.C.0/24 length, and 
some others are configured with, say, A.B.C.D/28 prefix length.

--
Pekka Savola                 "You each name yourselves king, yet the
Netcore Oy                    kingdom bleeds."
Systems. Networks. Security. -- George R.R. Martin: A Clash of Kings
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