[ppml] Increase the flexibility of IP allocations to facilitate planning

Leo Bicknell bicknell at ufp.org
Wed Jul 23 10:30:54 EDT 2003

In a message written on Tue, Jul 22, 2003 at 11:09:05PM -0400, jlewis at lewis.org wrote:
> Interesting example, but I question its relevance and completeness. 
> First, for every leased line customer you hook up, (assuming Frame, T1, T3,
> etc.) you're likely to also burn up a /30, and though only 2 IPs are being
> used, I'd count all 4 as used since you can't do anything with the first and
> last IPs in the /30.

These are in fact counted.  If you notice, 6 IP's in a /29 is 100%,
the customer needs 6, you must have a network and broadcast IP, so
that's all 8 in use, 100%.  There are two oddball cases at the
start, 1 IP and 2 IP's, the reality is if your customer needed
either amount you'd probably assign a /30 and count it as 50% or
100%.  Anyway, for anything > 3 I've counted the network and broadcast
towards utilization, and the previous cases are 100% moving the
average up, so I think I'm being as generous as possible.

Now, since you do have all these /30's that are "100%" they will
skew my assumed random distribution and push the average up slightly.
However it really isn't going to move much.

> That's going to skew the % utilization higher...but
> more importantly, AFAIK, ARIN only cares how much of your IP space you've
> used/assigned/allocated...not how much of the space is actually in use by
> devices, as long as you're being reasonable (following ARIN guidelines) with
> your assignment of space to customers...so in your example, the 75% average
> is irrelevant.  If you've assigned >=80% of your IP space to customers or
> internal use and can justify the assignments, you qualify for another
> assignment.

Not quite.  If you get a /23 (to make numbers easier) and allocate
4 customers /25's, but each customer only needed 4 IP's your not
going to pass ARIN's muster, even though you've "assigned" > 80%
of your IP space.  The rules are transitive, and apply down stream
as well.  If you assign IP's to a customer, they must also meet the
80% rule.

Problem is, if you have a customer with 7 devices (say, a small
office) you have to assign them a /28 for their lan, at 56.2%
utilization (7 + 2) / 16.  Now, what happens if you get a /23, and
end up with with 32 customers, each with 7 devices.  You've "assigned"
100% of your IP space, but only 56% of your IP space is in use.  By
the letter of the rules I don't belive you meet the 80% rule, yet
you can't do any better.

Again, I'm not suggesting this is a major issue.  For small allocations
like these the company with 7 devices is quietly encouraged by the
ISP to check off a box that says "14 IP's", since the block below
it was "6 IP's."  The ISP puts in the records submitted to ARIN
customer wanted 14, gave them a /28, all is happy.

I point this out only because people toss out that meeting 80% is
"trivial", and while for most larger ISP's dealing with a diverse
customer base this is true, there are definitely some corner cases
that cause people trouble.  I won't suggest they are lying, abusing
the system, or otherwise cheating, but it's very much the case that
they have to tell a particular version of the truth to pass the

       Leo Bicknell - bicknell at ufp.org - CCIE 3440
        PGP keys at http://www.ufp.org/~bicknell/
Read TMBG List - tmbg-list-request at tmbg.org, www.tmbg.org
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