Using /126 for IPv6 router links

Mark Smith nanog at 85d5b20a518b8f6864949bd940457dc124746ddc.nosense.org
Sat Jan 23 21:06:50 CST 2010


On Sat, 23 Jan 2010 20:55:52 -0600
Brandon Galbraith <brandon.galbraith at gmail.com> wrote:

> Sometimes good enough > perfect
> 
> Never know what is going to come along to turn your addressing plan on its head.
> 


It seems to me that what this really is about is trying to be in the
best position in the future. I think mainly it's about trying to avoid
unexpected and future renumbering/change of prefix length costs.

Possible positions or situations are :

1. you use a variety of node address lengths across your network, and
there are no future consequences - everything works and continues to
work

2. you use a single node address length (i.e. /64) across your network,
and there are no future consequences - everything works and continues
to work

3. you use a variety of node address lengths, and you'll have to
renumber to /64s, because you encounter unacceptable issues e.g.
device performance issues, inability to use features you'd find useful
e.g. SEND.

4. you use a single node address length, and you'll have to move to
variable length node addresses, because the IPv6 address space ends up
not being as big as it was designed and calculated to be.


Ideally, situations one 1. or 2. will occur, as they're the least
costly. 1. is both initially and operationally slightly more costly than
2. as you'll also have to also accurately manage prefix lengths, and
consider and/or address other non-/64 issues identified in RFC3627,
which I think makes 2. the better choice.

The question is, which of those two has the least risk of
devolving into the corresponding 3. or 4? As the addressing
architecture documents for IPv6 currently state that for other than
addresses that start with binary 000, the interface ID are required to
be 64 bits in length, it seems to me that situation 2. is the least
risky and least likely to devolve into situation 4. Vendors/developers
using RFCs as authoritative IPV6 documents are going to assume /64s, as
are future protocol developers.


> -brandon
> 
> On 1/23/10, Larry Sheldon <LarrySheldon at cox.net> wrote:
> > On 1/23/2010 8:24 PM, Owen DeLong wrote:
> >> On Jan 23, 2010, at 4:52 AM, Mathias Seiler wrote:
> >>> In reference to the discussion about /31 for router links, I d'like
> >>> to know what is your experience with IPv6 in this regard.
> >>>
> >>> 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 ? ;)
> >>>
> >> Use the /64... It's OK... IPv6 was designed with that in mind.
> >>
> >> 64 bits is enough networks that if each network was an almond M&M,
> >> you would be able to fill all of the great lakes with M&Ms before you
> >> ran out of /64s.
> >
> > Did somebody once say something like that about Class C addresses?
> >
> >
> > --
> > "Government big enough to supply everything you need is big enough to
> > take everything you have."
> >
> > Remember:  The Ark was built by amateurs, the Titanic by professionals.
> >
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> > 	
> >
> >
> 
> 
> -- 
> Brandon Galbraith
> Mobile: 630.400.6992
> FNAL: 630.840.2141
> 




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