AS Number Change on 1 January 2009

Posted: Thu, 24 Jul 2008

ARIN reminds the community that on 1 January 2009, all new Autonomous System Numbers (ASNs) issued will be four byte by default, unless otherwise requested. Following a globally coordinated policy, ARIN and all the Regional Internet Registries began allocating four-byte ASNs by request in January 2007; January 2009 marks the transition to allocating four-byte ASNs by default (see

Network operators may find themselves with a new four-byte ASN that upstream providers, peer networks, and customers cannot recognize. Without timely support from vendors, network operators risk having routers and network administration systems that will not accept the expanded four-byte number format. As such, ARIN urges operators to verify their vendors' routers will support four-byte ASNs.

To help vendors understand how to provide four-byte ASN support and to help network operators find products that support four-byte ASNs, APNIC has set up a special website at

Please contact if you have any questions concerning the ASN policy and four-byte ASNs.

Leslie Nobile
Director, Registration Services
American Registry for Internet Numbers (ARIN)