IPv6: numbering of point-to-point-links

Marco Hogewoning mch-nanog at xs4all.nl
Mon Jan 24 07:10:48 CST 2011

> 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.



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