/127 between routers?

[Mark Smith]

On Tue, 5 Jan 2010 20:26:55 +0100
Gert Doering <gert at space.net> wrote:

> Hi,
> 
> On Tue, Jan 05, 2010 at 08:59:58AM -0800, Michael K. Smith - Adhost wrote:
> > We use /64's for interfaces/interface sets/ and route /48's through and
> > to those interfaces.  [..]
> > Granted, I do use /128's for loopback addresses.
> 
> Same here.  For similar reasons.  Eventually, CGAs and SEND might show
> up, and then I might want /64s - and since a single /48 will be sufficient
> to number all routers we'll ever have, I stopped doing my IPv4 worries
> ("oh, this is all so wasteful") here.
> 
> I wouldn't want to enter a religious war here, though.  /64s might be
> useful one day, or might be not.  /120 and /112 work as well.  So pick
> something you're comfortable with.
> 

Why doesn't anybody put a price on managing multiple prefix lengths?
Maybe because they're so used to paying it with IPv4 that they don't
recognise it as a cost?

I'm for 'less is more'. /64s everywhere is less complexity and with
a /48 as an individual/end-site, you get 65536 of them. As an ISP with
a /32 you get 65K /48s, and if you use four of those for your own
internal addressing you'll have 256K /64s - how many ISPs have that
many internal point-to-point or otherwise links and loopbacks?

Why be conservative when the corresponding cost is to to record,
maintain and constantly have to work with and get right, differing
prefix lengths? If it's a /64 or nothing, you can never get it wrong.

Personally I'm considering a slight exception to the above by
using /128s on router etc. loopbacks, however that is for the
convenience of being able to see device IPv6 addresses/IDs by filtering
route table output e.g. "show ipv6 route | inc /128". That's a doubling
of the addressing management cost over just using /64s everywhere,
which is why I'm still debating it a bit.

I think the key realisation I've come to with IPv6 is that you get
enough address space that you don't just have to worrying about
allocating it out to make things work, you can allocate it out to also
make addressing *convenient* to work with, with as simple as possible
usually also being most convenient. Addressing convenience isn't a
property we've had with IPv4 for a long time (e.g. slicing up a Class B
at the 3/4th octets, using the 3rd octet as subnet numbers). With IPv6
we get it back, and we get more of it than we ever had with IPv4.

Regards,
Mark.