An Autonomous System Number (ASN) is a unique identifier for a network operator on the internet. It is used in routing to show which organization announces and manages a set of IP prefixes. For analysts, an ASN helps group infrastructure that may belong to the same provider, customer, or upstream network.
ASNs matter in cyber security because attackers often rotate domains and individual IP addresses, but they may keep using the same hosting or telecom networks. Defenders can pivot on ASN reuse to cluster command-and-control servers, identify shared infrastructure, and spot patterns that survive IP churn. ASN data is most useful when combined with DNS records, TLS certificates, and netblock analysis. It supports investigation and defense, but it is not proof of malicious activity on its own; it is a correlation point that helps build a clearer infrastructure picture.