Subject: RE: IPv6 Addr/Prefix clarification

From: "Michel Py" <[email-masked]>
Date: Sat, 26 Jan 2002 11:35:51 -0800

kre,

>> Brian E Carpenter wrote:
>> Nevertheless, it is not architecturally forbidden to subnet at
>> /124 if you really want to

> Robert Elz wrote:
> No, and it can be a good thing to do

No, but I am not so sure about it being a good thing.

> - if I have a lot of P2P links that I am going to number as 1 and
> 2 at the two ends (for lots of reasons I don't want unnumbered most
> times)
> If I have just a couple of dozen of them, then sure, burning numbers
> out of the subnet number space between /48 and /64 should be fine.
> But if I have hundreds, or perhaps even tens of thousands, then using
> some of those 62 padding 0's to number all of these P2P's, and
> consuming just one subnet number for the lot of them makes a lot of
> sense.

Id does make sense in IPv4. However, IPv6 has been designed with an
explicit trade-off between simplicity and allocation efficiency.

Let me make a comparison with Novell IPX: an IPX address is 80 bits,
which are 32 bits for the network, and 48 bits for the host (actually,
the MAC address most of the time). So, a /32 in IPX would be the
equivalent of a /64 in IPv6 (vaguely).

There is a Novell animal called the IPX internal network, which is
basically a loopback interface that is routable.

I have never heard anyone complaining that the IPX internal net, which
uses an entire "subnet" for only one network address, was a problem
because it was wasting address space.

> The answer to the original question is that nothing should assume
> anything about the internal structure of the 128 bits, except where
> actually required by some standard (such as auto-conf).

I concur that it would not be wise to assume anything, but saving IPv6
addresses does not strike me as good idea if it brings more complexity
and does not bring anything else than allocation efficiency.

Michel.

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From: Robert Elz <[email-masked]>
Date: Thu, 31 Jan 2002 15:09:08 +0700

Date:        Sat, 26 Jan 2002 11:35:51 -0800
    From:        "Michel Py" <[email-masked]>
    Message-ID:  <[email-masked]>

  | I concur that it would not be wise to assume anything, but saving IPv6
  | addresses does not strike me as good idea if it brings more complexity
  | and does not bring anything else than allocation efficiency.

I probably wouldn't either, though we don't want to totally forget
allocation efficiency - the way we make sure that IPv6 never runs out
is to always make sure we justify every allocation (which doesn't mean
organisations need to justify their need for a /48 - but I'd certainly be
making any organisation asking for a 2nd one (in the same aggregatable
block, I'm not talking about multi-connecting here) justify their use
of the first /48 they were allocated before giving away another.

But not adding any explicit length limits doesn't add complexity, it
reduces it.

kre

---

From: Peter Bieringer <[email-masked]>
Date: Thu, 31 Jan 2002 20:55:16 +0100

--On Thursday, January 31, 2002 03:09:08 PM +0700 Robert Elz
<[email-masked]> wrote:

>   | I concur that it would not be wise to assume anything, but
> saving IPv6   | addresses does not strike me as good idea if it
> brings more complexity   | and does not bring anything else than
> allocation efficiency.
> 
> allocation efficiency - the way we make sure that IPv6 never runs
> out is to always make sure we justify every allocation (which
> doesn't mean organisations need to justify their need for a /48 -

> but I'd certainly be making any organisation asking for a 2nd one
> (in the same aggregatable block,

If this will happen, which organisation would this be? Compared with
current IPv4 /48 with the 16 bits for SLA is similar to a IPv4 Class
A network.

Which organisation (not ISPs) uses more than one Class A network
*full* at the moment internally (means > 65k subnets)?

People always ask for as much as address space as possible, but in
IPv4 world it was more because of the number of clients and less the
number of possible subnetworks (thinking in Class A space).

        Peter