ASN

An ASN (Autonomous System Number) is a globally unique number that identifies an autonomous system — a set of IP addresses and prefixes operated under a single routing policy, typically by an ISP, cloud provider or large enterprise. ASNs are assigned by the regional internet registries RIPE NCC, ARIN, APNIC, LACNIC and AFRINIC, which in turn receive blocks from IANA.

ASNs are what tie internet routing together. When an operator announces its prefixes via BGP (Border Gateway Protocol), the ASN is inserted into the AS-PATH, so that other networks can see which autonomous systems traffic will traverse on its way to a destination. The AS-PATH is used both to prevent loops and to choose between multiple available paths.

Between autonomous systems, two main relationships are common: peering, where two networks exchange traffic directly and often settlement-free, and transit, where one AS purchases full internet reachability from a larger AS. Many organizations combine multiple transit providers and peerings for redundancy and performance.

ASNs were originally 16-bit (0–65535), but since 2007 32-bit ASNs (up to 4294967295) are also used. Private ASNs are reserved for internal use: 64512–65534 in the 16-bit space and 4200000000–4294967294 in the 32-bit space. Well-known public ASNs include Cloudflare (AS13335), Google (AS15169) and Amazon (AS16509). A single organization may hold multiple ASNs, often for different regions or subsidiaries.

An ASN lookup maps an observed IP address back to the AS that announces the covering prefix, which is how operators identify the network responsible for traffic, locate abuse contacts and confirm hosting providers. An ASN therefore identifies a routing domain — not a single IP address and not an ISP as a brand name.