Using /126 for IPv6 router links

Nathan Ward nanog at
Mon Jan 25 00:29:21 UTC 2010

On 24/01/2010, at 5:28 PM, Leo Bicknell wrote:

> In a message written on Sat, Jan 23, 2010 at 01:52:21PM +0100, Mathias Seiler wrote:
>> I use a /126 if possible but have also configured one /64 just for the link between two routers. This works great but when I think that I'm wasting 2^64 - 2 addresses here it feels plain wrong.
>> So what do you think? Good? Bad? Ugly? /127 ? ;)
> I have used /126's, /127's, and others, based on peers preference.
> I personally have a fondness for /112's, as it gives you more than
> 2 addresses, and a DNS bit boundary.
> For all the pontification about how there are enough /64's to number
> all the grains of sand, or other nonsense, I think that ignores too
> much operational information.
> rDNS is important, and becomes harder in IPv6.  Making it easier
> is importnat.
> Having a scan of a /64 fill your P2P T1 is poor design, all because
> you assigned 2^64 addresses to a link that will never have more
> than 2 functional devices.
> Most importantly, we should not let any vendor code any of these
> into software or silicon, in case we need to change later.

I too prefer /112s. I can take the first /64 in any assignment or allocation and set it aside for networking infrastructure.
The first /112 is for loopbacks, the remaining /112s are for linknets.

Then I can filter this /64 at my border, and it's easy.

You can do the same thing with /64 linknets, but then you have to set aside a block of them, and that might get hard if you have a /48 or something. Maybe not. What if you have a /56?

Maybe there is some value in linknets being effectively disposable so you never have to worry about problems coming from re-use. A single /64 full of /112s gives you 281 trillion.

For links to customers and other networks, I like /64s, because they are right now the standard so you're not going to run in to compatibility problems. If you've got links to customers you should have a /32, so setting aside a /48 or a /44 or something for those customer links is no huge drama.

Nathan Ward

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