Press Release

For Immediate Release
24 July 2008

Marissa Ramey

AS Number Change Could Affect Internet Routing From 1 January 2009

Chantilly, VA – Several Regional Internet Registries (RIRs), including the American Registry for Internet Numbers (ARIN), have warned that routers and network management software should be upgraded ahead of the increased distribution of four-byte Autonomous System (AS) numbers.

AS numbers are a vital part of the Internet’s core routing system, the Border Gateway Protocol (BGP). With existing two-byte AS numbers predicted to run out in early 2011, RIRs will issue four-byte AS numbers by default (unless otherwise specifically requested) beginning 1 January 2009, as the next phase of a transition from two- to four-byte numbers. Following a globally coordinated policy, RIRs began allocating four-byte AS numbers by request only in January 2007; January 2009 marks the transition to allocating four-byte AS numbers by default.

Without timely support from vendors, network operators risk having routers and network administration systems that won’t accept the expanded four-byte number format. As such, the RIRs urge operators to verify their vendors’ routers will support four-byte AS numbers.

Geoff Huston, Chief Scientist at APNIC, expressed severe concerns over failure to prepare for four-byte AS numbers: “AS numbers are often used to identify external relationships, set routing attributes, and manage traffic. Learning from our current experiences with IPv6 preparation issues, Internet engineers designed four-byte AS numbers to be backward compatible with much of the installed network infrastructure. But new entrants and networks that are expanding or merging will need new AS numbers and as of 1 January 2009 these will be, by default, four-byte AS numbers. If router software and support systems in critical parts of the Internet's infrastructure aren't upgraded by January, we’ll encounter some significant network routing problems. We’re extremely concerned that a lot of routers and network management software out there cannot and will not be able to recognize four-byte AS numbers.”

Raymond A. Plzak, President and CEO of ARIN, further added, “Network operators should be checking with vendors now to make sure they’re ready for four-byte ASNs. If they’re not, they should be planning to get capable routers as soon as possible.”

An Autonomous System (AS) is a collection of networks, or routers, administered as a group sharing a common set of routing policies, each defined with a unique number, or AS number.  Massive Internet growth has depleted the existing pool of two-byte AS numbers (65,536 numbers in total).  As a result, engineers have expanded the AS number space from two bytes to four bytes, to include over 4 billion AS numbers. Some routers do not currently support the use of four-byte AS numbers.

To help vendors understand what they need to do to provide four-byte AS number support and to help network operators find products that support four-byte AS numbers, APNIC has set up a special website at

About the American Registry for Internet Numbers

ARIN is the nonprofit corporation that manages the distribution of Internet number resources – IPv4, IPv6, and Autonomous System numbers – in its service region, which includes Canada, many Caribbean and North Atlantic islands, and the United States. More information on IPv6 adoption is available at and

The five RIRs are:

AFRINIC - Africa

APNIC – Asia-Pacific region

ARIN - Canada, many Caribbean and North Atlantic islands, and the United States

LACNIC - Latin America and parts of the Caribbean

RIPE NCC - Europe, Parts of Asia, and the Middle East

Each RIR performs a range of critical functions including:

  • The reliable and stable allocation of Internet number resources (IPv4, IPv6 and AS number resources)
  • The responsible storage and maintenance of this registration data
  • The provision of an open, publicly accessible database where this data can be accessed

RIRs also provide a range of technical and coordination services for the Internet community.