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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.

RIR Warns To Upgrade Routers

Regional Internet Registries (RIRs) have warned that routers and network
management software should be upgraded ahead of the increased
distribution of 4-byte AS numbers. Press Release

AS Number Resources

Why 4-byte AS Numbers
Autonomous System (AS) numbers are an integral part of the Border Gateway Protocol (BGP), why do we need to change them?


4-byte AS Number Specifications
The specification of how BGP has been altered to support 4-byte AS numbers is described in an IETF Internet Standard specification. Links to the Specifications

BGP and AS Number Backgrounder
If it is all a little confusing, you might find this beginner's guide to Autonomous System numbers a good place to start.

Operational Implications
Does everybody have to upgrade their routers? What about you? If you don't need to use 4-byte AS numbers do you need to upgrade?
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