[arin-ppml] Rationale for /22

William Herrin bill at herrin.us
Mon Jul 27 12:03:56 EDT 2009

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

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.

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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