Free ASN Lookup
Look up any Autonomous System Number or IP address. Get organisation, IP ranges, country, and routing information.
Autonomous Systems: How the Internet Is Actually Organised
The internet is not one network but tens of thousands of independent ones, each called an autonomous system: a collection of IP prefixes under a single administrative entity with a unified routing policy. Every AS is identified by its Autonomous System Number — AS13335 is Cloudflare, AS15169 is Google, AS16509 is Amazon — assigned by one of the five Regional Internet Registries. When you ask "who owns this IP?", the honest technical answer is "whichever AS announces the prefix that contains it".
Autonomous systems learn how to reach each other through the Border Gateway Protocol. Each AS announces to its neighbours the prefixes it can deliver traffic to, those neighbours pass the announcements on with their own ASN prepended to the path, and every router assembles the result into a global routing table currently approaching a million IPv4 routes. Routing decisions prefer the most specific matching prefix, then the shortest AS path — which is why an ASN lookup against live BGP data tells you who is really originating an address block right now, not just who registered it years ago.
Security teams lean on ASN data constantly. It turns an anonymous IP in a log into an attributable network — "this scan came from a bulletproof hoster in AS-X" is actionable in a way a bare address is not. It separates datacenter, VPN, and cloud traffic from genuine residential connections, which matters for fraud scoring and bot detection. And it powers coarse-grained controls: blocking one abusive ASN covers every prefix it announces, while allowlisting AS15169 keeps Google's crawlers reachable no matter which specific IPs they use this month.
Reading an ASN Lookup Result
ASNThe Autonomous System Number itself, e.g. AS13335. A 16- or 32-bit identifier assigned by a Regional Internet Registry to a single network operator.
OrganisationThe legal entity registered as the holder of the ASN — Cloudflare, Inc. for AS13335, Google LLC for AS15169. Sourced from RIR WHOIS data.
RIRWhich regional registry allocated the ASN: ARIN, RIPE NCC, APNIC, LACNIC, or AFRINIC. A rough signal of where the network is administered.
Announced prefixesThe IP ranges the AS originates in BGP. Large clouds announce hundreds of prefixes; a small regional ISP may announce only one or two.
PeeringThe neighbouring ASNs a network exchanges routes with, at exchanges or via transit. Peering data shows how well-connected a network is and how traffic reaches it.
Well-known ASNsAS13335 (Cloudflare), AS15169 (Google), AS16509 (Amazon AWS), AS8075 (Microsoft) — worth recognising on sight when reading logs or firewall alerts.
Frequently Asked Questions
What is an ASN?
An ASN (Autonomous System Number) is a unique identifier assigned to networks that control IP address routing on the internet. Examples: AS13335 (Cloudflare), AS15169 (Google), AS16509 (Amazon AWS).
What can I learn from an ASN lookup?
An ASN lookup reveals the organisation that controls the network, the IP ranges announced, the country of registration, and which RIR (ARIN, RIPE, APNIC, etc.) assigned it.
How do I find the ASN for an IP address?
Enter the IP address in the lookup tool — we will resolve it to its ASN automatically using BGP routing table data and WHOIS databases.
Why do security tools care about ASNs?
ASNs are used in threat intelligence to block or allow traffic from entire network ranges. Many organisations block ASNs known for hosting malicious traffic or bulletproof hosting.
Can two companies share one ASN?
Not normally — an ASN identifies a single administrative entity with a unified routing policy, and registry policy expects one organisation per autonomous system. What you do see is the reverse: one company operating many ASNs (Amazon runs AS16509, AS14618, and others), or small networks with no ASN at all because their upstream provider announces their address space for them. After mergers, acquired networks often keep announcing under legacy ASNs for years, which can make WHOIS organisation data look inconsistent.
How does an IP address map to an ASN?
Through BGP announcements: an autonomous system originates routes for the prefixes it controls, and those announcements propagate through the global routing table. An ASN lookup finds the most specific announced prefix covering your IP and reports the AS that originates it. Because the mapping comes from live routing data rather than a static registry, it reflects what the internet actually believes right now — including, occasionally, hijacked or mis-originated prefixes.
What do the Regional Internet Registries actually do?
The five RIRs — ARIN (North America), RIPE NCC (Europe and the Middle East), APNIC (Asia-Pacific), LACNIC (Latin America), and AFRINIC (Africa) — allocate IP address blocks and ASNs to networks in their regions, maintain the WHOIS databases recording who holds which resources, and operate the RPKI infrastructure used to cryptographically validate route origins. They do not control routing itself; they are the registry layer that BGP data gets checked against.
Why does ASN matter for firewall rules and geo-blocking?
Filtering by ASN operates at the network-owner level, which is far more robust than maintaining thousands of individual IP rules: allow AS15169 and you cover Google's crawlers even as specific addresses change; block a bulletproof hosting ASN and every prefix it announces is covered at once. ASN data also distinguishes datacenter, VPN, and residential traffic — a US IP announced by a hosting ASN carries very different risk from one announced by a residential ISP, a distinction plain geolocation misses entirely.
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