What Is an ASN? Understanding Autonomous System Numbers

An Autonomous System Number (ASN) is a unique identifier assigned to a collection of IP prefixes under the control of a single organisation — typically an ISP, cloud provider, university, or large enterprise. ASNs are the building blocks of internet routing.

Why ASNs Exist

The internet is not a single network; it is tens of thousands of independently operated networks (called autonomous systems) that agree to exchange traffic. The Border Gateway Protocol (BGP) uses ASNs to identify these networks and propagate routing information between them.

When your browser connects to a server, the packet traverses multiple autonomous systems. Each AS announces its IP prefixes to its peers via BGP, and routers use the ASN graph to determine the best path.

ASN Format

ASNs were originally 16-bit numbers (1–65535), giving a pool of about 64,000 ASNs. As the internet grew, 32-bit ASNs were introduced (RFC 4893), extending the space to over 4 billion. Today you will see both formats — for example, AS15169 (Google) or AS396982 (Google Cloud).

What the ASN Lookup Tells You

The ResolvIP /ip/{ip}/asn endpoint returns the ASN record for any IP address:

GET https://api.resolvip.net/ip/8.8.8.8/asn

{
  "asn": "AS15169",
  "org": "Google LLC",
  "isp": "Google LLC",
  "cidr": "8.8.8.0/24"
}

Key fields:

asn — the AS number in AS12345 format.
org — the registered organisation name.
isp — the ISP or network operator (may differ from org for sub-allocations).
cidr — the IP prefix (CIDR block) that contains the queried IP.

Practical Uses of ASN Data

Fraud and Abuse Detection

Residential ISP ASNs (e.g., Comcast, BT, Deutsche Telekom) are expected for genuine consumer traffic. Hosting and cloud provider ASNs (AWS, Hetzner, OVH) appearing in your login logs can indicate automation or scraping. Combine ASN data with the full enrichment endpoint (/ip/{ip}) to get proxy detection flags and cloud provider information.

Traffic Analytics

Grouping traffic by ASN gives a cleaner picture than grouping by individual IP. You can quickly identify which ISPs or cloud providers represent the bulk of your users, bots, or attackers.

Rate Limiting by Network

Instead of rate-limiting a single IP (which an attacker can rotate away from), rate-limit the /24 prefix or the entire ASN. This is far more effective against distributed attacks that still originate from a single cloud provider.

Where Does ASN Data Come From?

ASN-to-IP mapping is derived from BGP routing tables published by route collectors such as the RIPE Routing Information Service (RIS) and RouteViews. Geolocation providers combine these tables with WHOIS/RDAP allocation records to produce the ASN lookups you get from APIs like ResolvIP.