Daniel Karrenberg

16-bit ASN Exhaustion, Some Data

Daniel Karrenberg

We look at the consumption rate of 16-bit ASNs and try to make an educated guess on how long they would last if the current allocation rules would be extended.


The RIPE NCC has been issuing 32-bit (also known as four-byte) AS Numbers by default, unless otherwise requested, since 1 January 2009. However, uptake of 32-bit ASNs has been slow and many 32-bit AS Numbers have been returned.

We look at the consumption rate of 16-bit ASNs and try to make an educated guess on how long they would last if the current allocation rules would be extended.



The Straightforward Approach

The RIRs publish statistics on IP and AS number delegations on a daily basis. For ASNs, the records in these files have the  following format:


Taking the latest data from each RIR will show when each ASN was allocated. Naively, one could think that a simple cumulative plot, showing the number of ASNs in the latest stats files as a function of the allocation date would provide the desired information. However, this is not the case. Managing ASN allocations is a dynamic process. RIRs not only hand out AS numbers, they also receive back previously issued AS numbers, returned by the respective ASN holders; reasons for this are a.o. merger of networks (two Autonomous systems merged into one), bad debts leading to LIR closure etc. After some quarantine period, the returned ASNs become available for reallocation.

Thus, each stats file provides a snapshot of the current state. For each ASN it records the latest allocation date. Because an RIR stats file does not contain information on how many ASNs were returned over time, it provides an incomplete picture of the depletion rate of the free pool.

The ASNMIA analysis of 2005 showed the fraction of returned ASNs started to have measurable effects on the depletion rate in 2005. The increase in demand for new ASNs was offset by handing out recycled ASNs. Although the latest allocation date suggested slight exponential growth, the net increase at that time was still linear, thanks to this recycling.

A Better Approach

At any point in time, we define the size of the free pool as follows:

Free = Total ASN16 space  -  special purpose blocks  -  RIR allocations   


Total ASN16 space = 65536
special purpose blocks = 1042 (private AS + documentation purposes + ASTRANS)

Because the RIRs only publish daily snapshots, where allocation dates have changed due to recycling, we use the data published in historic stats files as an estimate for the RIR allocations. I.e. for each time t , with t > 2004-01-01 , the quantity <total RIR allocations> is defined as the total count of 16-bit ASNs seen in the collective RIR stats files for that time. Note that in this definition the free pool also includes those ASNs which have been returned, but are not yet available for reallocation because of a quarantine period or because they are still referenced. (for example, for RIPE NCC we see on 2009-05-05 the /ncc/ip-reg/as file lists 245 ASNs as REFERENCED ... NOT AVAILABLE ). This produces the following graph:


The clear outliers are caused by missing stats files for some dates.

Extrapolation and "Exhaustion Date"

Microsoft Excel's graphing package includes options to add trendlines to data series. Using a polynomial fit on the daily values from 2004-01-01 to 2009-08-14, the ASN16 pool would be exhausted around January 2014:


Only Most Recent Recent Data

One could argue that the fit above includes too many historic data points. Since the Internet has grown, lower allocation rates of 4, 5 years ago would overestimate the projected lifetime of the remaining ASN16 pool. To address this, we also made a polynomial fit to the data series of the last 3 years, from 2006-01-01 to 2009-08-14. This fit projects exhaustion by the middle of 2013:




Research and Analysis :  René Wilhelm

Editing : Daniel Karrenberg

This analysis was first presented briefly at RIPE 58. The graphs were updated with August 2009 data.




You may also like

View more

About the author

Daniel Karrenberg Based in Western Europe, NL&DE mostly

>>>>>>>>>>>> https://www.ripe.net/about-us/press-centre/publications/speakers/daniel-karrenberg <<<<<<<<<<<< Ample information about his past sins can be found using your favourite search engine. Following are a few additional keywords you might use, arranged by decade: 1980s: GUUG EUUG EUnet unido mcvax cwi RARE iepg RIPE; 1990s: RIPE+NCC rir iana postel terena ebone centr k.root-servers.net; 2000s: dnsmon nsd ris internet+society rssac; 2010s: ripe+labs ripestat ripe+atlas

Comments 3