An Autonomous System (AS) is a network under a single administrative control, such as an ISP, a university or a cloud provider. An Autonomous System Number (ASN) is the identifier used to route traffic between these networks.
ASNs are 32-bit numbers, often written like AS15169 (which belongs to Google). Knowing the ASN of an IP tells you which organization operates that block, which is useful for network and security work.
Our IP Lookup and ASN WHOIS tools surface ASN information from public intelligence providers.