ASN (Autonomous System Number)

An ASN is a numeric identifier for a network that announces internet routes under one routing policy. ASN stands for Autonomous System Number, and internet routers use it in the Border Gateway Protocol to describe which networks originate and carry address prefixes.

Looking up an IP address’s announced prefix and ASN can show whether it belongs to a hosting company, consumer internet provider, mobile carrier, university, or another network operator. Proxy detection and fraud systems use that classification as one signal. A large cluster of automated requests from a hosting ASN may be treated differently from traffic announced by a consumer ISP.

ASN data describes routing ownership and relationships, not the person or device using an address. Organizations can operate several autonomous systems, lease address space, or announce prefixes for customers. Registration labels can be stale or broad. Classification should therefore combine routing data with address registration, observed behavior, and current geolocation rather than assuming that one number proves how a connection is used.

ISP proxies rely on addresses associated with consumer providers, while datacenter proxies use hosting ranges. Can a residential proxy be detected? explains how ASN classification fits with other signals.

Related terms: ISP proxy and datacenter proxy.