The Internet Is a Network of Networks
The internet is not one network but tens of thousands of independently operated networks — internet service providers, cloud platforms, universities, content delivery networks and large companies. Each of these networks is called an autonomous system (AS), and each is identified by a unique Autonomous System Number (ASN).
For example, AS15169 is Google, AS13335 is Cloudflare and AS16509 is Amazon. You can look up any of these with our ASN Lookup tool.
How ASNs Are Used: BGP
Autonomous systems exchange routing information using the Border Gateway Protocol (BGP). Each network announces the blocks of IP addresses (called prefixes, such as 8.8.8.0/24) that it can deliver traffic to. Those announcements travel from network to network, and each one carries the list of ASNs it has passed through — the AS path. Routers use the AS path to choose routes and to avoid loops.
Because of this, the ASN that originates an announcement is a reliable way to identify which network is operating an IP address today.
Who Assigns ASNs?
ASNs and IP address blocks are distributed by five Regional Internet Registries (RIRs):
- ARIN – North America
- RIPE NCC – Europe, the Middle East and parts of Central Asia
- APNIC – Asia-Pacific
- LACNIC – Latin America and the Caribbean
- AFRINIC – Africa
Originally ASNs were 16-bit numbers (up to 65,535). As they ran out, 32-bit ASNs were introduced, so you will also see large numbers such as AS2xxxxxxxx. Some ranges are reserved for private use inside organizations and never appear on the public internet, for example 64512–65534.
How to Find Who Owns an IP Address
- Find the ASN: enter the IP address in our IP Location tool. The result shows the ASN and the organization that operates the network.
- Look up the ASN: open the ASN in the ASN Lookup tool to see the registered holder, the country, the registry and how many IPv4 and IPv6 prefixes the network announces.
- Check the registration: the RIR’s WHOIS data for the IP block often names a more specific customer, such as the hosting company’s client.
Owner vs. User
The ASN tells you which network routes the address, not necessarily who is using it. If an address belongs to a cloud provider such as AWS or a hosting company, the actual user is one of that provider’s customers. Only the provider can link the address to a specific customer.
Reporting Abuse
If an IP address is sending spam, scanning your servers or hosting phishing pages, report it to the network operator, not to the RIR. Our Abuse Contact Lookup finds the published abuse mailbox for an IP address. A good report includes the IP address, exact timestamps with time zone, and relevant log lines. Keep it factual and short; abuse desks handle large volumes.
Other Practical Uses of ASN Data
- Troubleshooting: a traceroute becomes much easier to read when you know which network each hop belongs to.
- Security: many firewall and fraud tools can block or rate-limit entire ASNs that are known sources of abuse, such as certain bulletproof hosting networks.
- Verifying DNS answers: comparing the ASNs of two DNS answers is how our China Firewall Test separates DNS poisoning from normal CDN behavior.