4-Byte AS Number Policy
This policy proposal nominates three dates for changes to the current AS Number allocation policy for the registry:
On 1 January 2007 the registry will process applications that specifically request 4-byte only AS Numbers and allocate such AS Numbers as requested by the applicant. In the absence of any specific request for a 4-byte only AS Number, a 2-byte only AS Number will be allocated by the registry.
On 1 January 2009 the registry will process applications that specifically request 2-byte only AS Numbers and allocate such AS Numbers as requested by the applicant. In the absence of any specific request for a 2-byte only AS Number, a 4-byte only AS Number will be allocated by the registry.
On 1 January 2010 the registry will cease to make any distinction between 2-byte only AS Numbers and 4-byte only AS Numbers, and will operate AS Number allocations from an undifferentiated 4- byte AS Number allocation pool.
No other changes in AS Number allocation policy are implied by this proposal.
Recent studies of AS Number consumption rates indicate that the existing 2-byte pool of unallocated AS Numbers will be exhausted sometime in the period between 2010 and 2016, absent of any concerted efforts of recovery of already-allocated AS Numbers  . Standardisation work in the IETF has produced a document that is currently being submitted as a Proposed Standard that will expand the AS Number space to a 4-byte field .
It is noted that some advance period may be required by network operators to undertake the appropriate procedures relating to support of 4-byte AS Numbers, and while no flag day is required in the transition to the longer AS Number field, it is recognised that a prudent course of action is to allow for allocation of
these extended AS Numbers well in advance of an anticipated 2-byte AS Number exhaustion date.
This policy proposal details a set of actions and associated dates for RIR AS Number allocation policies to assist in an orderly transition to use of the 4-byte AS Number space.
The essential attributes of this policy proposal are to facilitate the ease of transitional arrangements by equipment vendors, network managers and network operations staff, to provide the industry with some predictability in terms of dates and associated actions with respect to registry operational procedures for AS Number allocations.
It is proposed to identify 4-byte AS Numbers using a syntax of <high order 16 bit value in decimal>.<low order 16 bit value in decimal>. Accordingly, a 4-byte AS Number of value 65546 (decimal) would be identified as "1.10".
"2-byte only AS Numbers" refers to AS Numbers in the range 0 - 65535
"4-byte only AS Numbers" refers to AS Numbers in the range 1.0 - 65535.65535 (decimal range 65,536 - 4,294,967,295)
"4-byte AS Numbers" refers to AS Numbers in the range 0.0 - 65535.65535 (decimal range 0 - 4,294,967,295)