IPv6: numbering of point-to-point-links

[Crist.Clark at globalstar.com (Crist Clark)]

>>> On 1/24/2011 at  5:18 AM, <bmanning at vacation.karoshi.com> wrote:
> On Mon, Jan 24, 2011 at 02:10:48PM +0100, Marco Hogewoning wrote:
>> > While reading up on IPv6, I've seen numerous places that subnets are now
>> > all /64.
>> > 
>> > I have even read that subnets defined as /127 are considered harmful.
>> 
>> RFC3627, with a lot of discussion in the IETF on this. See also 
> https://datatracker.ietf.org/doc/draft-ietf-6man-prefixlen-p2p/ 
>> 
>> > However while implementing IPv6 in our network, I've encountered several
>> > of our peering partners using /127 or /126 for point-to-point links.
>> 
>> I personally don't any benefit in using /126 subnets.
>> 
>> > What is the Best Current Practice for this - if there is any?
>> > 
>> > Would you recommend me to use /64, /126 or /127? 
>> > 
>> > What are the pros and cons?
>> 
>> >From an operational point of view there is a risk that be using /64 somebody 
> can eat away a lot of memory by either scanning or even changing addresses. 
> This is also described in the draft above...
>> 
>> I would personally recommend to at least always assign the /64, even if you 
> would decide to configure the /127. RFC 3627 has been around long enough that 
> you will keep running into equipment or software that won't like the /127. In 
> which case you can always revert back to /64.
>> This will also allow you to use easy to remember addresses like ::1 and ::2, 
> saving you the headache of a lot of binary counting.
>> 
>> Grtx,
>> 
>> Marco
> 
> this results in -very- sparse matrix allocation - which is fine, as long as you 
> believe that 
> you'll never run out/make mistakes.  personally, i've use /126 for the past 
> 12 years w/o any
> problems.
> 
> there was never supposed to be a hard split at /64 - it was done as a means 
> to simplify autoconfig.

All of the (mostly religious) arguments about /64 versus any
smaller subnets aside, I'm curious about why one would choose
/126 over /127 for P-to-P links? Is this some kind of IPv4-think
where the all-zeros and all-ones addresses are not usable
unicast addresses? This isn't true in IPv6 (of course, it's not
strictly true in IPv4 either). Is there another reason?
-- 

Crist Clark
Network Security Specialist, Information Systems
Globalstar
408 933 4387