What Is an ASN?

An Autonomous System Number (ASN) is a unique identifier for an autonomous system (AS): a network or group of networks run under one routing policy that speaks BGP with other networks on the internet. ASNs let routers know which organization advertises which IP prefixes.

How do ASNs work?

Large providers, hosting companies, enterprises, and universities obtain an ASN from a regional internet registry. They announce IP blocks to peers and upstreams using BGP. Each announcement is tied to an ASN so other networks can build a global routing picture.

Public ASNs appear in global routing. Private ASNs exist for special cases (for example single-provider setups) and are not meant for the public internet. When you look up an IP, its origin is often described as “AS12345 Example ISP,” which is that network’s ASN and name.

ASNs do not replace IP addresses. They group prefixes under an operator. Two servers in the same datacenter may share an ASN; a mobile user’s IP maps to a different ASN belonging to a carrier.

Why does this matter for click fraud and ad fraud?

Fraud detection uses ASN data as a reputation and context signal. Datacenter and bulletproof host ASNs behave differently from residential or mobile carrier ASNs. A burst of ad clicks from many IPs inside a small set of ASNs can indicate automation or purchased traffic.

ASN is one input next to device signals, velocity, and conversion quality. It helps answer whether traffic looks like everyday consumer ISP users or infrastructure rented for bots and proxies. That supports decisions aligned with how fraud is detected, how suspicious clicks are scored, and how click fraud investigations use network context alongside ad fraud patterns.

For more detail on internet numbering policy, see ARIN’s ASN guide.