Bjørn Mork bjorn at
Tue Dec 22 09:21:22 UTC 2015

Owen DeLong <owen at> writes:
>> On Dec 20, 2015, at 08:57 , Mike Hammett <nanog at> wrote:
>> The idea that there's a possible need for more than 4 bits worth of
>> subnets in a home is simply ludicrous and we have people advocating
>> 16 bits worth of subnets. How does that compare to the entire IPv4
>> Internet?
> I have more than 16 subnets in my house, so I can cite at least one
> house with need for more than 4 bits just in a hand-coded network.
> Considering the future possibilities for automated topological
> hierarchies using DHCP-PD with dynamic joining and pruning routers, I
> think 8 bits is simply not enough to allow for the kind of flexibility
> we’d like to give to developers, so 16 bits seems like a reasonable
> compromise.

Thanks for summarizing why /48 for everybody is possible.  But I fear
that is not helping much against arguments based on "need". I believe it
is difficult to argue that anyone needs any IP address at all, given
that there are lots of people in the world who seem to survive just fine
without one...

So, with that sorted out, let's consider what you can do with 16 bits of
subnets.  One example is checksum neutral prefix translation (RFC6296)
without touching the interface id bits . Let's say you have two upstream
ISPs handing you the prefixes A/48 and B/56.  Neither offer any
multihoming support to residential users and both do BCP38 of course. So
you use B/56 internally and do prefix translation to allow your router
to select upstream without involving the clients.  Thanks to the A/48
from the first ISP, you are able to choose a set of 256 (or possibly 255
since 0xffff cannot be used) checksum neutral subnet pairs.

Yes, I know. Evil. No need. No CPE support.  Etc.

The important part is that 16 bits of subnets is enough to play
algorithmic tricks with the subnet part of your address too, whereas
this is much more difficult with fewer bits.  No, you don't need to do
it.  But you CAN.  The sparse IPv6 addressing model is about opening up
possibilities.  Note that those possibilities includes restricting
yourself to using a single address.  You don't have to use all your 2^80
addresses :)

And for the ISPs, using /48 for every user means fewer prefix lengths to
consider for routing and address management. Sure, we manage diverse
prefix lengths in IPv4 today, but why not take advantage of this
possible simplification if we can? Only those living on bugs will object
to simpler address databases and routing filters.


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