Introduction to Four-byte AS Numbers

In a situation similar to the exhaustion of IPv4 address space, the current pool of unassigned 2-byte Autonomous System (AS) numbers is predicted to run out.

While IPv6 is the industry's preferred solution to the IPv4 shortage, a new expanded system, which extends AS numbers to four bytes in length, is designed to address the AS number shortage.

Working cooperatively through the NRO (the Number Resource Organization) and in conjunction with ICANN (Internet Corporation for Assigned Names and Numbers) the world's five Regional Internet Registries (RIRs) are taking a coordinated approach to the transition from 2-byte to 4-byte AS numbers.

A major change in AS number management policy is for the RIRs to issue 4-byte AS numbers. Please refer to Section 6.3, "Timetable for moving from two-byte only AS numbers to four-byte AS numbers" in "Policies for Autonomous System number management in the Asia Pacific region".

Network builders may experience connectivity issues if their own equipment or the routing policies and hardware of their upstream providers is not compliant with this emerging standard.

To assist the Internet community to address these issues, share information about solutions and on-going issues, and to provide a forum for general discussion about the introduction of 4-byte Autonomous System numbers, ICONS is providing this information in the hope that other community members, including ISPs, users, and vendors can contribute up-to-date information for the benefit of all community members.