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. 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-bit (two octets) to 32-bit, which greatly expands the available pool of ASNs.