What Is an ASN (Autonomous System Number) and Why It Matters

An Autonomous System Number is a unique identifier assigned to a network that runs its own routing policy and connects to the internet through BGP. Your ISP has one. Google has several. Every major cloud provider, CDN, and telecoms carrier has them. Understanding ASNs is understanding the building blocks of how the internet routes traffic.

What an Autonomous System Actually Is

An Autonomous System (AS) is a network or group of networks under a single administrative and routing policy — effectively, a single organisation's portion of the internet. An ISP is an AS. A university with its own IP block and routing is an AS. Amazon Web Services runs dozens of ASes covering different regions and services.

Each AS is assigned a number — originally 16-bit (1 to 65535), now 32-bit to accommodate growth. ARIN, RIPE, APNIC, and the other regional registries assign ASNs. Currently there are over 100,000 active ASNs on the internet.

How ASNs Appear in IP Lookups

Every IP address belongs to an AS. An IP lookup will show you the ASN and the associated organisation name — AS15169 is Google, AS16509 is Amazon AWS, AS13335 is Cloudflare. For security analysis, this is immediately useful: an IP from AS16509 is from AWS, which means it's a cloud server, not a residential user. An IP from a residential ISP's ASN means something different entirely.

That's exactly why ASN data matters in practice. Fraud detection systems, email spam filters, and security tools all use it as part of their risk scoring. Cloud and VPS provider ASNs are higher risk for certain types of activity because they're so frequently used for automated attacks, bots, and spam. Residential ASNs get more benefit of the doubt.

ASNs in Threat Intelligence

Threat intelligence analysts use ASNs to cluster related infrastructure. Attackers who build out malicious infrastructure often use the same ASNs repeatedly — either because they prefer certain hosting providers that are less aggressive about abuse, or because they're buying IP addresses in the secondary market from specific sources. Seeing the same ASN in a series of attack campaigns is a meaningful clustering signal.

Blocking entire ASNs is also a common enterprise security tactic — if you have no business reason to receive traffic from a particular VPS hosting provider's ASN, blocking it at the firewall eliminates a significant fraction of automated attack traffic.

Peering: How Networks Connect to Each Other

Autonomous Systems connect to each other through peering relationships. Settlement-free peering is an agreement between two networks of roughly equal size to exchange traffic at no cost — each sends and receives traffic for the other's customers without payment. Transit is when a smaller network pays a larger network to carry its traffic to destinations it can't reach directly.

Internet Exchange Points (IXPs) are physical locations where many networks peer simultaneously. The Amsterdam Internet Exchange (AMS-IX) and the London Internet Exchange (LINX) handle hundreds of terabits per second of traffic daily through hundreds of member networks. When you connect to a Dutch website from the UK and the response comes back in 10ms instead of 100ms, it's probably because both networks peer at LINX and the traffic never left London.