3.7.b [ii] 4 bytes AS number
During the early time of BGP development and standardization, it was assumed that availability of a 16 bit binary number to identify the Autonomous System (AS) within BGP would have been more than sufficient. The 16 bit AS number, also known as the 2-byte AS number, provides a pool of 65,536 unique Autonomous System numbers. The IANA manages the available BGP Autonomous System Numbers (ASN) pool, with the assignments being carried out by the Regional Registries. 2 byte ASN is now regarded as historical.
A solution to this depletion is the expansion of the existing 2-byte AS number to a 4-byte AS number, which provides a theoretical 4,294,967,296 unique AS numbers.
The Cisco IOS BGP “4-byte ASN” feature allows BGP to carry a Autonomous System Number (ASN) encoded as a 4-byte entity.
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 3875-3882). . Kindle Edition.