In September 2013, the IANA issued its last remaining full block (1024) of 2-byte AS numbers, with 512 going to APNIC and 512 going to Ripe NCC. Of the 65,536 total AS numbers in the 2-byte pool, there now remain just 496.
Following a globally coordinated policy implemented in January 2007, the five Regional Internet Registries (RIRs) began allocating 4-byte AS numbers upon request. In January 2009, a new, replacement global policy went into effect that directed IANA and the RIRs to make no distinction between 2 and 4-byte AS numbers, and to issue them from a common pool. To date, thousands of 4-byte AS numbers have been issued collectively by the RIRs, but there are still reports of issues with the widespread deployment of these 4-byte numbers.
The NRO has been working in conjunction with ICANN on the transition from 2-byte to 4-byte AS numbers since 2007 and has publicly advised network operators to upgrade their routers and network management software in anticipation of the increased distribution of 4-byte AS numbers.
However, the RIRs are finding that there are still routers out there that do not support the use of 4-byte AS numbers, resulting in some customers coming back to the RIRs to exchange their 4-byte AS numbers for 2-byte AS numbers.
As the pool of 2-byte AS numbers is depleted globally, networks will need to rely on 4-byte AS numbers to identify the distinct autonomous networks that make up the Internet's core routing system. It has now reached a critical point where all network operators must prepare their networks to support 4-byte AS numbers or risk the successful growth of the Internet.