ASN (Autonomous System Number)
Updated
An ASN identifies a network that applies its own routing policy, and it is the unit between which BGP exchanges routes. Public ASNs are assigned by the regional internet registries; the ranges 64512 to 65534 and 4200000000 to 4294967294 are reserved for private use by RFC 6996.
In practice
A BGP session between two private networks usually works with private ASNs, and cloud interconnections let you choose a private ASN for your side.