v6 subnet size for DSL & leased line customers

Joe Greco jgreco at ns.sol.net
Sun Dec 23 22:45:51 UTC 2007

> 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. 

More likely, at least some of them are fairly naive questions.

For example, /experience/ tells us that corporate LAN policies are often
implemented without regard to what "we", as Internet engineers, would
prefer, so I can guarantee you with a 100% certainty that there will be
at least some networks, and more than likely many networks, where you
will not be able to simply request a prefix delegation and have that work
the way you'd like.  There will always be some ISP who has delegated, or
some end site who has received, a "far too close to being just large
enough" allocation, and so even if we assume that every router vendor
and IPv6 implementation from here to eternity has no knobs to disable
prefix delegation, simple prefix exhaustion within an allocation will be 
a problem.  All the screams of "but they should have been allocated more"
will do nothing to change this.

Further, if we consider, for a moment, a world where prefix delegation is
the only method of adding something like an Apple Airport to an existing
network, this is potentially encouraging the burning of /64's for the
addition of a network with perhaps a single client.  That's perfectly fine,
/as long as/ networks are allocated sufficient resources.  This merely
means that from a fairly pessimistic viewpoint, IPv6 is actually a 64-bit
address space for purposes of determining how much address space is

So, from the point of view of someone manufacturing devices to attach to
IPv6 networks, I have some options.

I can:

1) Assume that DHCP PD is going to work, and that the end user will have
   a prefix to delegate, which might be valid or it might not.  This leaves
   me in the position of having to figure out a backup strategy, because I
   do not want users returning my device to Best Buy because "it don't
   work."  The backup strategy is bridging.

2) Assume that DHCP PD is not going to work, and make bridging the default
   strategy.  DHCP PD can optionally be a configurable thing, or autodetect,
   or whatever, but it will not be mandatory.

I am being facetious here, of course, since only one of those is really
viable in the market.  Anyone who thinks otherwise is welcome to explain to
me what's going to happen in the case where there are no P's to D.

I will leave the difference between corporate and residential as an exercise
to the reader; suffice it to say that the answers are rather obvious in the
same manner.

... JG
Joe Greco - sol.net Network Services - Milwaukee, WI - http://www.sol.net
"We call it the 'one bite at the apple' rule. Give me one chance [and] then I
won't contact you again." - Direct Marketing Ass'n position on e-mail spam(CNN)
With 24 million small businesses in the US alone, that's way too many apples.

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