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

["Dunn, Jeffrey H." <jdunn at mitre.org>]

Mike,

Actually, you cannot just assign a /48 to each site.  The RIR H-ratio
requirements may make this infeasible.  Further, each /48 (/56 for
IANA) allocations must be registered with the RIR, which is an
administrate headache.  Finally, there is the issue of reverse lookup
registration for sites.  These are just the policy issues.

I think what Brian was trying to point up is that mapping the IPv6
sub-netting scheme to an existing IPv4 sub-netting scheme would be
easier of the number of low order bits below the prefix in an address
is the same.  For example, a IPv4 /24 with 8 low order bits below the
prefix might be mapped to a /120 IPv6 address.  Clearly, this could be
abstracted to a scheme using 2 or 4 x (32 - IPv4 prefix length) low
order bits in the IPv6 address, i.e., assign a /n, where n = 128 - 2 or
4 x (32 - IPv4 prefix length).

Best Regards, 
  
Jeffrey Dunn 
Info Systems Eng., Lead 
MITRE Corporation.
(301) 448-6965 (mobile)


-----Original Message-----
From: michael.dillon at bt.com [mailto:michael.dillon at bt.com] 
Sent: Tuesday, September 30, 2008 12:25 PM
To: briand at ca.afilias.info; Dunn, Jeffrey H.
Cc: alexandru.petrescu at gmail.com; ipv6 at ietf.org; rbonica at juniper.net;
Steve_Eiserman at ao.uscourts.gov; Pasi.Eronen at nokia.com; Sherman, Kurt
T.; draft-ietf-v6ops-addcon at tools.ietf.org; ralph.liguori at disa.mil;
night at nist.gov; dougm at nist.gov; v6ops-chairs at tools.ietf.org; Martin,
Cynthia E.
Subject: RE: what problem is solved by proscribing non-64 bit prefixes?

> 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.

Given that in IPv6, you can justify allocating a /48 to each
separate site, which gives you 16 bits to mirror the IPv4
subnet hierarchy, while maintaining 64 bit interface sddresses,
I don't see a technical issue here.

And I would really recommend that you upgrade all of your management
systems to fully support IPv6 instead of relying on tricks like
generating an IPv6 address by applying a transform to an existing
IPv4 address. Then you have no technical issue at all.

--Michael Dillon


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