IPv6: numbering of point-to-point-links

Ray Soucy rps at maine.edu
Mon Jan 24 15:20:33 CST 2011

The only advantage of a 126-bit prefix is if you're using it to take
advantage of the short address, and keep all your point-to-point
networks in the same address space so that you can easily identify

This is really only personal preference for network engineers who may
not want to be dependent on DNS and like to have key link addresses
committed to memory (we're only human, after all).

Prefix: 2001:DB8::/32

2001:DB8::4/126 (or 2001:DB8::5 and 2001:DB8::6)

Though, you could accomplish almost the same thing with using a 64-bit
prefix length:
2001:DB8:1::/64 (or 2001:DB8:1::1 and 2001:DB8:1::2

That said.  By not using the 64-bit boundary you may be sacrificing
performance optimizations with today's processors that lack operations
for values larger than 64-bits.

Either way is acceptable and is simply a matter of personal
preference.  Link networks do not contain a dynamic number of hosts,
so logically there is no reason to accommodate more addresses than
they originally call for.

RFC 3627 recommends against using a 127-bit prefix-length due to
potential (implementation specific) problems with DAD, and that is
enough reason to avoid it for most people.

Nobody is right.  Nobody is wrong.  It's just preference.

We have a lot of link networks and opted early on to make use of
126-bit prefixes for them because that worked nicely for our
allocation schema.

On Mon, Jan 24, 2011 at 7:48 AM, Lasse Jarlskov <laja at telenor.dk> wrote:
> Hi all.
> 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.
> However while implementing IPv6 in our network, I've encountered several
> of our peering partners using /127 or /126 for point-to-point links.
> 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?
> --
> Best regards,
> Lasse Jarlskov
> Systems architect - IP
> Telenor DK

Ray Soucy

Epic Communications Specialist

Phone: +1 (207) 561-3526

Networkmaine, a Unit of the University of Maine System

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