An Autonomous System Number (ASN) is a unique identifier assigned to an autonomous system — a network or collection of networks operating under a single, unified routing policy. ASNs are the identifiers BGP uses to track which organization originates or controls a block of IP addresses on the internet.
The internet is not a single network — it is tens of thousands of independently operated networks interconnected by agreement. Each independently operated network, or group of networks under common administrative control with a single routing policy, is called an autonomous system (AS). An ISP, a cloud provider, a university, or a large enterprise with its own IP address blocks can each constitute an autonomous system.
The key criterion is not size but routing policy: an AS presents a consistent view of its reachability to the rest of the internet. Internally, an AS may use any routing protocol it chooses. Externally, it speaks BGP to its neighbors.
ASNs were originally defined as 16-bit unsigned integers, giving a range of 0 to 65535. As internet growth exhausted the 16-bit space, RFC 6793 extended ASNs to 32 bits (sometimes called 4-byte ASNs), greatly expanding the available pool. A 32-bit ASN is written in dotted notation (e.g., 1.1000) or as a plain decimal (e.g., 65536).
Within the 16-bit range, a block of numbers is reserved for private use — for ASNs used internally within an organization or lab, not intended to appear in the public internet routing table. An analogous private range exists in the 32-bit space. These are analogous to RFC 1918 private IP addresses: useful for internal routing without consuming globally unique numbers.
When an AS announces a route to a neighbor, BGP prepends its own ASN to the AS_PATH attribute. As the announcement propagates across the internet, each AS that forwards it appends its ASN. The result is a list that records the exact sequence of autonomous systems a route has traversed:
64496 64511 15169
In this example, the route originated in AS 15169 and passed through AS 64511 and AS 64496 before reaching the current AS. The AS_PATH serves two purposes: it is used in best-path selection (shorter paths are generally preferred) and it prevents routing loops (an AS will not accept a route whose AS_PATH already contains its own ASN).
The AS that originates a given IP prefix is visible in the BGP routing table. Several public looking glasses and tools allow you to query this. Practically, the most common methods are:
Knowing the ASN behind an IP helps in several operational situations: identifying the network responsible for a particular hop in a traceroute, investigating whether traffic is routing through an unexpected third-party network, correlating an IP address with a known provider or CDN, and understanding RPKI validation results — which are always expressed in terms of prefix-to-ASN bindings.
Look up any ASN with the BGP tool
Open BGP Lookup