Re: ISP customer assignments

[William Herrin <herrin-nanog () dirtside com>]

On Mon, Oct 5, 2009 at 2:10 PM, Brian Johnson <bjohnson () drtel com> wrote:
> What would be "wrong" with using a /64 for a customer who only has a
> local network? Most home users won't understand what a subnet is.

It's a question of convenience... your customers', but more
importantly yours. Every time you have to deviate from your default,
whatever default you pick, that's an extra overhead cost you have to
bear. Absent a compelling reason not to, you should structure your
default choice so that it accommodates as many customers as possible.

There are too many good reasons why someone might want to use two
subnets with two different security policies and not enough reasons
(zero in fact) why it would help you to give them less subnets than
the 16 in a /60.


> So a customer with a single PC hooked up to their broad-band
> connection would be given 2^64 addresses?
> I realize that this is future proofing, but OMG! That’s the IPv4
> Internet^2 for a single device!

Some clever guy figured out that if you use 64 bits you can write
algorithms that automatically assign an interface's IP address based
on its MAC address without having to arp for it. Since the details of
IPv6 were not yet firmly fixed at that point and ram is cheap, why not
add an extra 64 bits for that very convenient improvement? This is
called "stateless autoconfiguration."

Some even more clever guy figured out that if the first clever guy's
strategy is used, it becomes a trivial matter to track someone
online... based on the last 64 bits of their IP address which will
remain static for the life of the hardware they use regardless of
where they connect to the 'net. Given this rather blatent weakness and
given that you still need DHCP to assign DNS resolvers and the like,
stateless autoconfiguration will probably end up being a waste. That's
unfortunate, but look at it this way: the important part is not how
many addresses are wasted, it's how many addresses are usable.

Regards,
Bill Herrin




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William D. Herrin ................ herrin () dirtside com  bill () herrin us
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