[ppml] IPv6 addressing, allocation, and subnets

michael.dillon at bt.com michael.dillon at bt.com
Mon Nov 19 04:43:55 EST 2007

> michael.dillon at bt.com wrote:
> >> It is time to get with the CIDR program, not to advocate the 
> >> antiquated class-ful paradigm.
> >>     
> >
> > CIDR is IPv4. We are discussing IPv6 which is different.
> No, I'm afraid you're wrong on this point.
> CIDR is classless inter-domain routing. It is neither IPv4 nor IPv6.

You were talking about subnet planning, not about routing. 
In the context of addressing plans and planning network architecture,
CIDR is only important with IPv4. CIDR is what allows you to design
subnets with the minimum number of addresses and aggregate the mess
behind a shorter prefix.

In the IPv6 world, CIDR is not terribly relevant even if, technically,
it still exists. The fact is that you *CAN* design your IPv6 subnetting
plan along classful lines if you want to.

> With it, covering aggregates and variable-length subnet masks 
> (VLSM) are possible, and provide the basic tools for scaling 
> both internal and external routing domains.

Notice how infrequently people mention VLSM these days because it
is just considered to be a part of CIDR. The meaning of terminology
changes depending on context and depending on the times. To most
networking people these days, CIDR is an IPv4 thing that lets you
make your subnets as small as possible to conserve addresses. The
term is tainted with the concept of "address conservation" which
is not necessary or desirable when planning IPv6 networks.

> And, as it happens, IPv6 *is* CIDR-oriented.

Technically, IPv6 doesn't need CIDR to be tacked onto the
design because it was there in the first place. IPv6 always
has routed based on the longest prefix.

> So, your contention that IPv6 is "different", and not CIDR, 
> is patently false.

You are mincing words. 90% of the people on this list have never
read any of these RFCs and don't care to debate the fine points 
of technical terminology. This is a policy forum. We don't really
care whether or not IPv6 can be stretched this way or that. We 
have to respect the IETF's core design and make our policies around

--Michael Dillon

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