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To map an IP address to an ASN, find the BGP route prefix that covers the address, then read the route’s origin autonomous system number (ASN). RIPEstat and Team Cymru offer lookup options. Treat the result as a routing observation—not proof of an IP’s legal owner, the internet provider serving a particular person, or that person’s location.
What an IP-to-ASN lookup tells you
An autonomous system (AS) is a group of IP networks operated under a single, clearly defined routing policy, as described in RIPE NCC’s ASN policy document. Its ASN identifies that system in routing exchanges; it is not an IP address or a geographic coordinate.
IP-to-ASN mapping typically identifies the origin ASN for the BGP prefix covering the queried address. A prefix is a block of addresses advertised together in routing. The returned prefix matters: it shows which route the ASN is associated with, rather than implying that an address has one permanent, individually assigned owner. Team Cymru’s explanation describes its origin lookup in these terms: mapping an IP or prefix to a corresponding BGP Origin ASN.
Keep three questions separate:
- Routing: Which ASN originates the route covering this address in the data a service observes?
- Registration: Which organization or registration record is associated with the address block or ASN?
- Geolocation or user service: Where might an address be located, or which provider serves an end user?
An ASN lookup directly addresses the first question. Registration records and geolocation can provide other context, but neither is interchangeable with a routing observation.
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Look up one IPv4 or IPv6 address
Use RIPEstat
- Open RIPEstat and enter the IP address in its query field.
- Inspect the routing-related results for the covering prefix and origin ASN.
- Record the address, prefix, ASN, source, and time of the lookup. The prefix helps you interpret the result as a route observation.
RIPEstat accepts queries for IP addresses, prefixes, ASNs, ranges, hostnames, and country codes, and provides both an interactive interface and an API. Its overview is available from RIPE NCC.
Use Team Cymru’s lookup page
- Open Team Cymru IP to ASN Lookup.
- Submit the IP address and inspect the returned ASN and route context.
- Keep IPv4 and IPv6 addresses in separate queries: the page says it supports both families but a single query cannot mix them.
The lookup page’s behavior and interface can change. If you depend on it operationally, confirm the current instructions on the page rather than relying on older examples.
Automate repeated lookups
RIPEstat Data API
RIPEstat describes its Data API as the interface underlying its UI. Consult its current documentation for the appropriate endpoint, parameters, and response format before implementing a client. The documentation says the API is suitable for non-commercial use and that commercial users should contact RIPE NCC; verify current terms before deploying a commercial integration. The documentation also notes that freshness depends on collection frequency, update intervals, processing delays, failures, and caching.
Team Cymru DNS-based origin queries
Team Cymru documents DNS-based lookups in which an IPv4 address’s octets are reversed and the origin lookup zone is appended; the TXT response includes an ASN and route prefix, among other fields. The available example is reproduced in a 2016 presentation, not a current official service specification. Because DNS zone syntax and behavior must be current to be useful, verify the exact query format against Team Cymru’s current documentation before putting it into a production script. Do not treat the historic example as guaranteed current syntax.
Maintain your own routing data
A self-maintained routing dataset or BGP feed can support local lookups, but it shifts the work to you: acquiring data, refreshing it, and operating the collection pipeline. RIPE NCC’s historical guidance discusses live BGP feeds and offline datasets as approaches, but a local copy is only as current as its known collection and update schedule. See RIPE NCC’s guidance on abuse-contact information sources for that historical context.
Choose a lookup method for the job
| Method | Useful for | Important consideration |
|---|---|---|
| RIPEstat interface | Interactive queries and inspection of routing context | Results reflect the data sources and update behavior described in RIPEstat documentation. |
| Team Cymru lookup page | Interactive IP-to-ASN lookup for IPv4 or IPv6 | Do not mix address families in one query; confirm current page behavior. |
| RIPEstat Data API | Automating queries through RIPEstat | Check current endpoints, response details, freshness, and commercial-use terms. |
| Team Cymru DNS method | DNS-oriented automation where current syntax is confirmed | The cited example is a 2016 reproduction; verify current syntax before relying on it. |
| Local BGP data or feed | Operating lookups within a self-managed data workflow | You must know and manage its collection and refresh schedule. |
When comparing services, check what routing data they expose, whether they show a prefix and observation time, how often their inputs update, whether they cache answers, and how they handle IPv4 and IPv6. For APIs, also check terms and operational requirements. These factors can matter more than whether two lookup pages present the answer in different formats.
Interpret differences and avoid common mistakes
A different ASN can reflect a different observation
Two services may disagree because their viewpoints, source datasets, update timing, processing delays, or caching differ. For an outage, security investigation, or disputed route, note the timestamp and covering prefix, then compare another routing source. A single cached answer is not timeless proof of a route.
An ASN is not necessarily the end-user’s ISP
The origin ASN identifies the network announcing the covering route in the data observed by the lookup service. It does not establish which retail provider serves an individual user or device. Networks can carry traffic for customers or downstream networks, so describe the result as a routing origin unless you have independent evidence for a more specific relationship.
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For registration information, query the relevant RDAP or Regional Internet Registry data. RFC 9910 specifies RDAP searches for IP network and ASN registration objects. A registry record answers a different question from which ASN originates a route; use the appropriate data for the claim you need to make.
Geolocation is a separate lookup
An ASN result alone does not place an address in a city or country, nor does it prove where a person using it is located. Use a geolocation source if location is the actual question, and treat that source’s result according to its own methodology and uncertainty.
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If the goal is to capture a lookup page rather than parse routing data, ScreenshotNeo offers a website screenshot API. One GET request returns an image or PDF; its clean-shot workflow accepts consent banners and removes supported consent platforms, newsletter popups, and chat widgets before capture. Bot checks, blank pages, failed loads, and cache hits are not billed, and responses include verdict and billing headers. Its MCP server gives AI agents tools to take screenshots, get page information, and capture PDFs.
For a screenshot of a lookup page, replace the example URL with the page you need. See the ScreenshotNeo API documentation for options and response handling.
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://asn.cymru.com/ -o shot.webp
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Troubleshoot a lookup
- No result or an unexpected answer: Check for a typo, confirm whether the input is IPv4 or IPv6, and retry with another source. Compare the returned prefix and lookup time, not just the ASN.
- IPv4 and IPv6 batch rejected: Split the addresses by family for Team Cymru’s lookup page; it says one query cannot intermingle them.
- Automated response differs from the web page: Check the API’s current documentation and the query parameters or response fields you use. RIPEstat’s Data API and interface should not be assumed to have identical update timing.
- DNS query fails: Do not copy the 2016 reproduced example blindly. Verify the current Team Cymru syntax and service behavior before troubleshooting your DNS client.
- Local data looks old: Find the dataset’s collection and refresh timestamps. If no schedule is known, do not label the result current.
- Commercial API use is planned: Confirm RIPEstat’s current terms and contact RIPE NCC as its documentation directs for commercial use.
Sources and scope
This procedure describes IP-to-ASN mapping through routing-origin data. RIPEstat documentation states it was last updated 20 March 2025; service interfaces and terms can change, so consult the linked current documentation when implementing an integration. The Team Cymru interface’s address-family limitation is stated on its lookup page; the DNS-format example cited above comes from a 2016 secondary reproduction and should be independently verified before production use.
Frequently Asked Questions
Can an IP address have more than one possible ASN result?
Yes. Different data sources or observation times can report different routing views, so preserve the source, timestamp, and covering prefix with the result.
Does an ASN identify a specific person or device?
No. It identifies an autonomous system in routing, not an individual subscriber, device, or user location.
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