v6 subnet size for DSL & leased line customers

Joel Jaeggli joelja at bogus.com
Sun Dec 23 01:14:59 UTC 2007

Randy Bush wrote:
> Joel Jaeggli wrote:
>> Randy Bush wrote:
>>> the "but what if they want the toaster on a separate subnet from the
>>> blender" gives a new depth to 'reaching.'  the one case i can think of
>>> for firewalling/routing within the home is to keep the bathroom scale
>>> from locking the fridge.
>> If ipv6 subnetting is going to be hosed up at this point it's going to
>> be done by people deploying it.
> unfortunately, 'hosed up' only seems to be understood some years out.
> smb's point is apt, we always end up too small.
> but i still have a very hard time understanding what we are gonna do
> with more than a /56 to a consumer connection.

Leave enough address space for pd to occur? We know that if I hand you
the end-user a /64 that the first device that you connect to the network
 (wireless/ethernet router) will nat because it wants a l3 boundary
between the outside and the inside for v6 just like it has for v4. If
that device, when it boots and requests pd receives a /64, cool... but
what does it do when some device downstream from it asks for address space?

So is a /64 ok for an end customer? No because it doesn't meet the
criterion of a location where one and only one subnet will be needed.
Does it need a whole /48? Probably not.

What you need in your provisioning system and how you structure
address-space usage for the benefit of your IGP is that downstream
devices need to be able to request and receive blocks of a size
commiserate with their needs without increasing the footprint of your
routing table.

> and if i start to go to the left of a /56, where do i stop?  there is no
> obvious detent on the knob.

There is a huge detent at /48, but there's a certain amount of guidance
that can only be derived from operational experience. It's not clear to
me why /56 would be unacceptable, particularly if you're delegating them
to a device that already has a /64. Are one's customers attached via
point-to-point links, or do they sit on shared broadcast domain where
their cpe is receiving a /128 and requesting pd from the outset?

When someone plugs an apple airport into a segment of the corporate lan
should it be be able to request pd under those circumstances as well?
how is that case different than plugging it in on a residential connection?

These are issues providers can and should grapple with. Much as
assigning a /32 to a residential customer vs /30 or /28 is a business
decision. In many if not most cases we don't currently provide as many
v4 addresses as there are devices within the customer premises. Enough
addresses isn't going to be an issue in v6, The dynamic creation of
topology that was automatically and at least in one direction
transparently created by restricted-cone nats is obviously something new.

> randy

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