Another v6 question

Mark Andrews marka at
Fri Jan 28 03:25:28 CST 2011

In message <AANLkTinDQdH5Z==mbYvm-OstA2m-WVkxo7vKyLc8x7vf at>, Per 
Carlson writes:
> Hi Owen.
> > The downside is that it doesn't provide enough bits for certain kinds of =
> auto-topology
> > management that are being considered by CE vendors. I highly recommend /4=
> 8 instead.
> I've seen this claim (you need a /48) from your side several times,
> but never seen any explanation why a /56 won't work.
> Is there any requirement that sub-delegations must happen on 8-bit
> boundaries? AFAICS there is at least nothing in the RFC. Wouldn't for
> example a nibble boundary work equally well (splitting a /56 into 16
> /60s, each containing 16 /64s)?
> I don't challenge the claim, I'm just trying to understand the
> rationale behind it.

There is a model where the down stream CPE devices always request
powers of two prefixes.  It doesn't take many CPE devices daisy
chained to exhaust 8 bits.

The other model is to just request as many /64 as needed using
multiple requests with different identifiers.  You can daisy chain
out past the limits of IPv6 to route packets with that model.

Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka at

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