What Is an ASN and How to Look Up Who Owns an IP
Published 2026-11-03 · ipok.fyi
Every IP address on the public internet is reachable because some network tells the rest of the world, "send traffic for this range to me." That network is an Autonomous System, and its ID is the ASN (Autonomous System Number). If you want to know who is behind an IP — a home ISP, a mobile carrier, a cloud provider or a VPN company — the ASN is where you start.
This guide explains ASNs in plain language and shows how to look up an IP's owner for free with ipok.fyi.
What is an Autonomous System?
An Autonomous System is a group of IP ranges run by one organization under a single routing policy. ISPs, cloud providers, large companies, universities and content networks each operate one or more. They exchange routes with each other using BGP (Border Gateway Protocol), the protocol that glues the internet's networks together.
What is an ASN?
An ASN is the unique number assigned to an Autonomous System, written like AS15169 or AS16509. ASNs are allocated by the five Regional Internet Registries (RIRs) — ARIN, RIPE NCC, APNIC, LACNIC and AFRINIC — under IANA. Originally they were 16-bit numbers; 32-bit ASNs were introduced to provide many more.
Well-known examples:
- AS15169 — Google
- AS16509 — Amazon (AWS) — see AS16509 on ipok.fyi
- AS13335 — Cloudflare
- AS8075 — Microsoft
ASN vs IP address vs prefix
- IP address: one endpoint, e.g.
8.8.8.8. - Prefix: a block of addresses, e.g.
8.8.8.0/24. - ASN: the network that announces one or many prefixes.
One ASN can announce thousands of prefixes, and an organization may operate several ASNs for different regions or products.
Why look up an IP's ASN?
- Find the owner. The ASN's registered organization tells you whether an IP belongs to an ISP, a cloud host, or a business.
- Detect datacenter traffic. If the ASN belongs to a hosting provider, the IP is very likely a server, VPN or proxy rather than a home user. (See How to check if an IP is datacenter or residential.)
- Investigate logs and abuse. When an IP hammers your login page, the ASN shows which provider to report it to.
- Debug routing and geolocation. If your IP's location looks wrong, the ASN and its registry country often explain why.
- Allow- or block-listing. Some teams rate-limit or challenge traffic from specific hosting ASNs.
How to look up who owns an IP (step by step)
1. Check your own IP
Open ipok.fyi. The report shows your IP, its ASN, the organization name, and whether that ASN is on the hosting/datacenter list — plus how much it contributed to the risk score.
2. Check any other IP via API
curl "https://ipok.fyi/api/check?ip=1.1.1.1"
the JSON includes the hosting/ASN list match, the IP type, geolocation, and a factors list explaining the score. No API key is needed.
3. Browse the datacenter ASN directory
The ipok.fyi ASN directory lists hosting and cloud networks. Each page — for example /asn/16509 — explains that IPs from that network usually look like datacenter traffic to websites.
4. Cross-check with registry data
For legal or abuse purposes, confirm with the responsible RIR's WHOIS/RDAP service, which lists registered contacts and abuse addresses. On the command line:
whois 8.8.8.8
How ASN data is used in IP reputation
IP reputation tools rarely trust a single signal. A common pattern:
- Map IP → ASN.
- Classify the ASN (ISP, mobile, hosting, education, business).
- Add other evidence: reverse DNS hostnames, DNS blacklist hits, Tor exit lists, and browser consistency like timezone and WebRTC.
ipok.fyi follows this approach and shows each step, so you can see that, for example, 40 points came from "ASN is on a hosting/datacenter list" rather than receiving an unexplained number.
Common gotchas
- Shared infrastructure: CDNs and cloud networks host millions of unrelated customers. An IP inside AS13335 or AS16509 tells you the provider, not the website or person behind it.
- Resold space: small VPN and proxy companies often rent IPs from larger hosts, so the ASN shows the host, not the VPN brand.
- Mixed-use networks: some providers run both hosting and consumer broadband under one ASN, which makes classification harder.
- Changes over time: IP blocks are transferred and leased. Lists need regular updates.
FAQ
What does ASN stand for?
Autonomous System Number — the ID of a network that announces IP ranges on the internet via BGP.
Can I find a person from an ASN lookup?
No. An ASN identifies the network operator (for example, an ISP), not an individual subscriber. Only the ISP can map an IP to a customer, typically under legal process.
How do I know if an ASN is a datacenter?
Look at the organization (cloud and hosting companies are obvious) and check a curated list such as the ipok.fyi datacenter ASN directory.
Is ASN lookup free on ipok.fyi?
Yes. The IP check, /api/check?ip= API and ASN pages are free with no signup.
Why does my home IP show a business or hosting ASN?
Some ISPs register their ranges under a parent company, and some "residential" services are actually resold hosting. If your result looks wrong, check the reverse DNS and registry details shown in the report.
Look up an IP now: ipok.fyi · Datacenter ASN directory