[arin-ppml] Rationale for /22

Bill Darte BillD at cait.wustl.edu
Mon Jul 27 14:48:18 EDT 2009

Every effort to lower minimum allocations throughout the years has met
with resistance.  Each successful policy managed a 'bit at a time' to
ensure 'nothing bad happened'....

In recent years, there have been few calls for a further lengthening and
those that emerged gained little support.

Proposals are always welcome...

Bill Darte

> -----Original Message-----
> From: arin-ppml-bounces at arin.net 
> [mailto:arin-ppml-bounces at arin.net] On Behalf Of William Herrin
> Sent: Monday, July 27, 2009 11:04 AM
> Subject: [arin-ppml] Rationale for /22
> Question for y'all:
> What is the rationale behind a /22 minimum size for 
> multihomed organizations? Why not a /24?
> The reason behind /20 for single-homed orgs is fairly straightforward:
> an ARIN allocation adds a route to the BGP table which 
> wouldn't otherwise be needed. Routes are expensive and the 
> cost falls into overhead since it isn't recoverable directly 
> from the org announcing the route. And we're not really 
> certain how many routes we can handle before the network 
> falls over. So, we restrict the availability of 
> non-aggregable IP addresses to just very large organizations. 
> For smaller orgs, renumbering sucks but at least it only 
> costs the renumbering org, not everyone else.
> The reason behind nothing smaller than a /24 is also straightforward:
> many if not most ISPs filter out BGP announcements smaller than /24.
> There is tremendous inertia behind /24 as the minimum 
> backbone-routable quantity going back to the pre-CIDR days of 
> class-C addresses. So, an ARIN allocation smaller than /24 
> would generally be wasted addresses, unusable on the Internet.
> But why peg multihomed orgs at /22 instead of /24? 
> Multivendor multihomed orgs have to announce a route anyway, 
> regardless of whether the addresses are from an ISP or 
> directly from ARIN. Their routes are not aggregable, even if 
> assigned from ISP space. That's the way the technology works 
> and no new tech in the pipeline is likely to change it.
> With load balanced server clusters and NAT you can pack a 
> heck of a lot of punch into a multihomed /24 if you want to. 
> And as a community it's to our benefit to want registrants to 
> pack the maximum punch into their address space: IPv4 
> addresses are becoming scarce. So why do we restrict ARIN 
> assignments to folks who can write papers which justify a /22?
> Excluding conspiracy theories (the big bad ISPs want lock in) 
> I'd like to hear ideas, answers and even recollections from 
> folks who were there when the size was set as to why we 
> should prefer /22 as the minimum multihomed size assignable by ARIN.
> Regards,
> Bill Herrin
> --
> William D. Herrin ................ herrin at dirtside.com  bill at herrin.us
> 3005 Crane Dr. ...................... Web: 
> <http://bill.herrin.us/> Falls Church, VA 22042-3004 
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