subnet prefix length > 64 breaks IPv6?

Alexandru Petrescu alexandru.petrescu at
Thu Dec 29 14:51:02 UTC 2011

Le 28/12/2011 13:13, Ray Soucy a écrit :
> 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,

Well yes, but only if it's an address which doesn't start with 000 (3
zero bits).  I understand an address which starts with 000 can have an
interface id of length generic 128-n where n is prefix length. (RFC4291
"Addressing Arch", pp. 6,  1st par).

Generally speaking, my mind is disturbed by suggestions that the
Interface ID must always be precisely of length 64.  BEcause there is no
particularly valid reason to impose it so, other than the vaguely useful
and semantically doubtful 'u' bit - any software ever checks it on
reception?  At an extreme reading, it may look as the "secure" bit.



> 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.

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