ARIN to Issue 4-byte ASNs by Default Starting on 1 January 2009

Posted: Tue, 30 Dec 2008

Beginning on 1 January 2009, ARIN will be assigning four-byte Autonomous System Numbers (ASNs) by default, unless a 2-byte ASN is specifically 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

Because this is a relatively new implementation, it is possible that some devices may not be able to recognize the new 4-byte ASNs. ARIN urges operators to verify that their routers and those of any transit providers or peers are ready and able to accept 4-byte ASNs.

Additional information about four-byte ASNs or how to request them can be found 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)