Using /126 for IPv6 router links

Mark Andrews marka at
Wed Jan 27 05:51:05 UTC 2010

In message <20100127160401.1a963a56 at>, Mark Smith writes:
> Sure. However I think people are treating IPv6 as just IPv4 with larger
> addresses, yet not even thinking about what capabilities that larger
> addressing is giving them that don't or haven't existed in IPv4 for a
> very long time. People seem to be even ignoring the maths of how big a
> single /48 is, just in terms subnets. I've never worked on an
> individual network with 65K subnets (with the Internet being a network
> of networks), and I doubt many people on this list have. Yet people
> seem to treating a /48 as though all networks will have 65K subnets,
> and therefore they'd better start of using longer than /64s because
> they might run out of subnets.
> I care about this because I don't want to see people have to change
> their addressing in the future to /64s, because of that will typically
> involve a lot of out of hours work (which could include me if I
> inherit a network that has had longer than /64s deployed (there's my
> bias)). I'd prefer to see people go the other way - deploy /64s
> everywhere, as per the IPv6 Addressing Architecture, and if that proves
> to be the wrong case, then go to the effort of deploying longer
> prefixes. I think going from /64s to longer prefixes is far less likely
> going to be needed than the other way around.

And if you have more than 65K networks you have the justification
for a second /48 at which time you can decide whether to request a
/47 and renumber into it or just use two non-contiguous /48's.
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka at

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