estimation of number of DFZ IPv4 routes at peak in the future

William Herrin bill at
Tue Mar 8 21:55:19 CST 2011

On Tue, Mar 8, 2011 at 10:17 PM, Mikael Abrahamsson <swmike at> wrote:
> We had an interesting discussion the other day at work. We were speculating
> on how many DFZ IPv4 routes there would be at peak in the future before it
> starts to decline again due to less IPv4 usage.
> My guess therefore is a peak around 450-500k IPv4 DFZ routes and that this
> would happen in around 3-5 years. I wanted to record this for posterity.
> What is your guess, any why?

Five million.

Assuming the /24 boundary holds (this is likely) and we're only
carrying global unicast and anycast routes (1. to 223. excluding
RFC1918, 127, etc) the theoretical maximum number of possible IPv4
prefixes is around 28M. (2**24)*0.8~=14M /24's. Plus 7M /23s, 4M
/22's, etc.

However, practical issues will prevent excessive numbers of fully
covered prefixes... So we won't generally see both /24's under a /23.
We might see a /24 and a covering /23 but if we do we won't generally
see the other /24. This drops us to an upper bound of 14M.

There will also be a significant number of prefixes where there's just
no gain from breaking them up. You're using the entire /20 at your
site and you only have one /24 that you want routed differently than
"normal." This will pull it down still further, cutting it somewhere
between half and a third of the 14M upper bound.

It'll take 10 to 20 years to get there. If we're actually able to
start retiring IPv4 in 10 years then it'll peak lower. But if IPv4
sticks around, I think the global IPv4 BGP table will reach a steady
state somewhere around 5 million prefixes.

Bill Herrin

William D. Herrin ................ herrin at  bill at
3005 Crane Dr. ...................... Web: <>
Falls Church, VA 22042-3004

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