Autonomous System Numbers
Autonomous System Numbers (ASN) are globally unique numbers and significant part of the Internet routing architecture known as the Autonomous System. Autonomous System Numbers are taken from a 16-bit number field. There are reserved ASNs, including 0 and 65,535, which can be used to identify non-routed networks. ASNs starting from 64,512 up to 65,534 are reserved for private use while the ASN 23,456 is dedicated to be used in the ASN pool transition.[1] This means that the available quantity of ASNs for Internet routing is limited.
As per RFC4893 (updated by RFC6793) the ASN space has been extended from 16-bt (two octets) to 32-bit, which greatly expands the available pool[2] of ASNs.
ICANN (by means of the IANA-function) is responsible for distribution to RIRs