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More Number Scarcity

2 April 2012
By Leo Vegoda

Last year ICANN allocated the last five IPv4 blocks to the Regional Internet Registries (RIRs). Since then we have seen a concerted effort on the part of network and content providers to make sure they support IPv6, so they’ll be ready for the next few billion Internet users. But there’s another Internet number resource which is running short: 16-bit Autonomous System Numbers (ASNs).

Internet networks learn how to reach destinations (IP addresses) using an IETF protocol called the Boarder Gateway Protocol (BGP). BGP uses unique Autonomous System (AS) numbers to identify individual networks (routing domains) in order to announce the reachability of destinations (IP addresses). Originally, BGP used 16-bit numbers, allowing slightly more than 65,000 ASs

Internet growth in the 1990s made it clear that a 16-bit number space is insufficient and the first proposals for a 32-bit number space, allowing about 4.3 billion AS numbers, were published in 2001. In parallel with this work, the addressing community began developing a transition policy in the RIRs’ open policy forums and socialising it with network operators, the consumers of AS numbers.

The IETF work was published as a standards track RFC in 2007. And while IPv4 and IPv6 networks do not interoperate, networks that don’t know about 32-bit AS numbers can still communicate with networks using 32-bit AS number using a transition mechanism described in the RFC. The RIR community work was ratified as a Global Policy in 2008 and included a timetable for the transition. This echoed the timetables the addressing communities had agreed to in the policies governing AS number assignments in each of their regions.

These regional policies have promoted 32-bit AS number assignments and now require the RIRs to treat all AS numbers as part of the same 32-bit pool. Unfortunately, lots of networks reported problems making use of 32-bit ASNs. In the region served by the RIPE NCC, which has had the most success at assigning 32-bit ASNs [PDF, 2.66 MB] (PDF slides 8 & 9), they still comprise just a third of the total ASN assignments. The reason for the huge disparity in acceptance between the five regions is not clear. Earlier this month, John Curran, ARIN’s CEO & President, asked the ARIN community for an easy, straightforward answer to this question.

One of the reasons networks have problems deploying 32-bit AS numbers is that network providers who have not upgraded their equipment will see the same transition AS number being used by different network providers. This duplication of AS numbers causes problems for monitoring tools and even path selection mechanisms. But there are just three blocks of 16-bit ASNs left, so the time is rapidly approaching when networks won’t be able to swap out a 32-bit AS number for a 16-bit replacement AS number. There won’t be any new 16-bit AS numbers left.

Much has been written about the lack of IPv4 addresses and its impact on the potential economic growth of countries and industries.  The need to transition to IPv6 has triggered coordinated action plans from major network, content and access providers to support both IPv4 and IPv6.  However, little attention has been paid to the diminishing pool of 16-bit ASN numbers, which are another enabler of Internet growth.  The technical community has defined a 32-bit ASN specification; the addressing communities have implemented suitable assignment policies; what is now needed is widespread acceptance of 32-bit AS numbers by network providers.


Leo Vegoda