subnet prefix length > 64 breaks IPv6?

Ray Soucy rps at
Wed Dec 28 12:13:30 UTC 2011

On Wed, Dec 28, 2011 at 6:23 AM, Iljitsch van Beijnum
<iljitsch at> wrote:
> Also somehow the rule that all normal address space must use 64-bit interface
> identifiers found its way into the specs for no reason that I have ever been able
> to uncover. On the other hand there's also the rule that IPv6 is classless and
> therefore routing on any prefix length must be supported, although for some
> implementations forwarding based on > /64 is > somewhat less efficient.

This ambiguity has always bothered me.  The address architecture RFC
requires a 64-bit interface identifier, but it's required to be
unenforced by implementation, which makes it more of a suggestion at
best.  I think the wording should be updated to changed MUST to
SHOULD.  That said, and despite my own use of prefix lengths other
than 64-bit, I do believe that a 64-bit prefix for each host network
is in our long-term interest.  Not having to size networks based on
the number of hosts is a good thing.  Features made possible by a
64-bit address space is a good thing.

Ray Soucy

Epic Communications Specialist

Phone: +1 (207) 561-3526

Networkmaine, a Unit of the University of Maine System

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