# Using IPv6 with prefixes shorter than a /64 on a LAN

Owen DeLong owen at delong.com
Tue Jan 25 23:24:38 CST 2011

```> ...

>
> What did that just do to your per-site /64? That you have
> no hope of ever seeing a user use up? It just turned
> that /64 into a /112 (16 bits of port space, 32 bits
> of cloud identifier space.) What's the next killer app
> that'll chew up more of your IPv6 space?
>
Dude... You missed... It's not supposed to be a /64 per site.
The plan is a /48 per site. Yes, you managed to use one of
the subnets up pretty well... ON A SINGLE SUBNET.

Now, what do you do for the other 65,535 of them at the
one site?

> I'm all for IPv6. And I'm all for avoiding conjecture
> and getting to the task at hand. But simply assuming
> that the IPv6 address space will forever remain that -
> only unique host identifiers - I think is disingenious
> at best. :-)
>
Well.. There's assuming (like your assumption that a /64
per site was the original plan) and then there's doing
the math.

Even with the utilization you've mentioned above, my
math still holds.

Owen

>
>
>
> On Tue, Jan 25, 2011, Owen DeLong wrote:
>
>> I love this term... "repetitively sweeping a targets /64".
>>
>> Seriously? Repetitively sweeping a /64? Let's do the math...
>>
>> 2^64 = 18,446,744,073,709,551,616 IP addresses.
>>
>> Let's assume that few networks would not be DOS'd by a 1,000 PPS
>> storm coming in so that's a reasonable cap on our scan rate.
>>
>> That means sweeping a /64 takes 18,446,744,073,709,551 sec.
>> (rounded down).
>>
>> There are 86,400 seconds per day.
>>
>> 18,446,744,073,709,551 / 86,400 = 213,503,982,334 days.
>>
>> Rounding a year down to 365 days, that's 584,942,417
>> years to sweep the /64 once.
>>
>> If we increase our scan rate to 1,000,000 packets
>> per second, it still takes us 584,942 years to sweep
>> a /64.
>>
>> I don't know about you, but I do not expect to live long
>> enough to sweep a /64, let alone do so repetitively.
>>
>> Owen
>
> --
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