Small IX IP Blocks

Paul Stewart paul at
Sun Apr 5 11:29:57 UTC 2015


I worked for a provider until recently that happened to get an IP assignment
at an IXP that was transitioning from /25 to /24.  It was painful chasing
down peers to get them to change their netmask just so we could connect.
This went on for several months dealing with the peering/network contacts of
whom many of them didn't know the mask had changed in the first place.


-----Original Message-----
From: NANOG [mailto:nanog-bounces at] On Behalf Of Bill Woodcock
Sent: Saturday, April 4, 2015 10:36 PM
To: Mike Hammett
Cc: NANOG list
Subject: Re: Small IX IP Blocks

> On Apr 4, 2015, at 7:28 PM, Charles Gucker <cgucker at> wrote:
> I've been involved in IX renumbering efforts because exchange(s)
> decided to use /25's instead of /24's.    It's painful because
> troubleshooting can be a little difficult as differing subnetmasks are
> in play.   If you have the address space, use a /24.    ARIN has IPv4
> address space specifically reserved for the use by IXPs.

Yes.  Listen to Charlie.

We did a bunch of analysis on size of IXP subnets, and how it changes over
time, relative to the age of the IXP.  To summarize drastically, the first
/24 typically lasts about 15-18 years.  Only a tiny handful of exchanges
(less than 2%) are actually supporting more than 254 participants yet at
this point.  That number will continue to grow over time.  At the same time,
it's not worth the trouble of renumbering more than once in that time
period, so don't be penny-wise and pound-foolish by trying to use a /25,
particularly when ARIN hands out /24s to IXPs specifically to keep them from
running into that trap.


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