Free ASN Lookup Tool
Enter any Autonomous System Number to see the operator, country, RIR, all the IPv4 and IPv6 prefixes it announces in BGP, its peers and its upstreams. Works for 16-bit and 32-bit ASNs. The result is the same whether you call this an ASN lookup, an AS lookup, an autonomous system lookup or a BGP lookup. Free, no signup.
What an ASN Lookup Returns and How Complete Each Field Is
Coverage measured across the live BGP table from public route collectors (RIPE RIS, RouteViews) plus RIR registry data (ARIN, RIPE, APNIC, LACNIC, AFRINIC). Numbers refresh continuously.
BGP coverage
Live numbers across the global routing tableWhat an ASN Lookup Tells You
Every result returns the same set of fields, whether you call it an ASN lookup, an AS WHOIS or a BGP lookup:
- Operator name (organisation that runs the AS)
- Country of registration and the regional registry (RIR)
- Date the ASN was allocated
- Type of network (transit, content, hosting, enterprise, IXP, government, education)
- Full list of IPv4 prefixes announced in BGP, with size
- Full list of IPv6 prefixes announced in BGP
- BGP peers (other ASNs the operator exchanges routes with)
- Upstream transit providers (who the AS pays for default connectivity)
- RPKI signing state for the announced prefixes
- Abuse contact and technical contact from the RIR
What an ASN Lookup Will Not Tell You
The ASN identifies the network, not the customers on it. The lookup will not return:
- Names or addresses of individual subscribers
- Traffic volume or bandwidth for any specific IP inside the AS
- Customer cone (which downstream ASNs depend on this one for transit) without paid intelligence services
- Real-time route hijack alerts (use BGPMon or Cloudflare Radar for that)
The ASN is a wholesale identifier, not a retail one. For "what is the IP of user X behind this AS" you would need cooperation from the operator and a court order.
How to Look Up an ASN You Already Have
Paste an ASN into the box above. With or without the AS prefix works: AS15169 and 15169 both resolve to the same Google AS. For 32-bit ASNs the same rule holds (AS3320001 or 3320001). The result page lists the operator, the country, every prefix, and links every related ASN and IP for further drilling.
If you do not have an ASN yet but need to find one for a specific IP, use the IP lookup. The result there includes the announcing AS, with a direct link into this lookup for the prefix list. ASNs are written without a space (AS3356, not AS 3356), in capital letters by convention, but the lookup accepts any case.
BGP, Peering and Transit in Plain Words
Once two networks want to exchange traffic, they need a way to tell each other "I own these IP ranges, send my traffic this way". That conversation runs over BGP (Border Gateway Protocol), defined originally in RFC 1163 (1989) and currently RFC 4271 (2006). Every AS that participates in BGP runs at least one BGP router that announces its prefixes to neighbours and listens for their announcements. The collection of all those announcements is the global routing table, about 980,000 IPv4 entries today.
Two ASes can relate to each other in three ways:
- Transit: Customer pays provider for full reachability to the rest of the internet. Tier-1 carriers (Lumen, NTT, Telia, Cogent, Telstra, Tata, GTT) sell transit to everyone smaller.
- Peering: Two networks exchange traffic between their own customer bases for free. Saves money and improves latency, but only covers each other's prefixes, not the wider internet.
- IXP membership: Multiple networks plug into a shared switch at an Internet Exchange Point (DE-CIX Frankfurt, AMS-IX Amsterdam, LINX London) and peer in bulk over that fabric.
Our ASN result page shows peers and upstreams parsed from public route collector data. CDNs and large content networks (Google, Cloudflare, Akamai, Meta, Amazon) typically peer aggressively to keep traffic off paid transit. Small ISPs usually pay one or two upstreams and peer at one IXP.
RPKI, Route Origin Validation and Hijacks
BGP was designed to trust whatever its neighbours announce. That worked when the internet was a few research networks. It does not work when any AS in any country can announce someone else's prefix and steal their traffic. BGP hijacks happen routinely: Pakistan hijacking YouTube (2008), Russia's Rostelecom hijacking Mastercard (2017), Hetzner accidentally hijacking parts of Amazon (2022). The damage ranges from a few minutes of outage to credential theft if the attacker can intercept TLS handshakes.
The fix is RPKI (Resource Public Key Infrastructure). Operators publish ROAs (Route Origin Authorizations) cryptographically signed by their RIR, declaring "AS X is allowed to announce prefix Y". Other operators run Route Origin Validation on their routers and drop announcements that fail validation. Adoption has climbed steadily; about 42% of routes in the global table now have valid ROAs, and major networks (Cloudflare, Google, Amazon, Telia) drop invalid announcements at the edge. Our result page shows the RPKI state of each prefix.
When an ASN Lookup Is Actually Useful
Abuse investigation. An IP is hammering your service. The IP lookup gives you the geo and the ISP. Drilling into the ASN lookup tells you whether this is a major residential ISP (send abuse report to the listed contact, expect action), a hosting provider (report to their abuse desk, expect a customer suspension), or a bulletproof host (do not bother, just block the AS). The operator's listed abuse email is the place to send the complaint.
Network planning. Buying transit, choosing an IXP, picking a CDN POP. You want to know which networks are well-peered in the city you sit in. The peer list on each ASN page is the cheap version of the analytics that cost real money elsewhere.
BGP troubleshooting. A traceroute is dropping at a specific hop. Look up the ASN at that hop. Find their abuse / NOC contact. Send them the trace. Resolving cross-network issues is mostly a matter of finding the right human to talk to, and the ASN record is the directory.
Hijack monitoring. Compare today's announced prefixes for your own AS against yesterday's. If a prefix you announce starts also being announced by an AS you do not know, that is a hijack in progress. Public services like BGPMon and Cloudflare Radar alert in real time, but a manual check via this lookup is a free fallback.
Threat intelligence. Some ASNs concentrate disproportionate amounts of abuse: ransomware C2, brute-force, scanning, residential proxy networks. Security teams keep block-lists at the AS level for high-confidence bad actors. The ASN lookup tells you the type of network (hosting vs residential vs mobile) which is a useful pre-classifier.
Geolocation troubleshooting. An IP geolocates to "the country the AS is registered in" by default. If your customer in Berlin gets served the Bangkok version of your site, the IP is probably announced by an AS whose registered country is wrong. Compare the AS country to the geolocation country on the IP lookup; if they disagree, the AS-level data is usually the truth.
Compliance and licensing. Restricted content rules often map to AS rather than IP. "Do not serve to AS owned by company X" is a simpler policy than maintaining IP block-lists. The lookup gives you the owner identity for the rule.
More About How ASNs Work and How the Internet Routes
Below are the questions and details people most often ask after running an ASN lookup. Skim what is interesting, skip the rest.
What is an Autonomous System
An Autonomous System (AS) is a network or collection of networks under a single administrative authority, with a single routing policy. ISPs, hosting providers, large enterprises, universities, and big content companies each run one or more ASes. Each gets a unique ASN from its regional registry. Networks too small to need their own ASN connect through a larger provider's AS, paying for IP transit.
16-bit vs 32-bit ASNs
The original ASN format was 16-bit, allowing 65,536 values (0 to 65,535). Numbers 64,512 to 65,534 are reserved for private use. The remaining ~64,000 public values ran low by 2007, and IANA introduced 32-bit ASNs (RFC 4893) which start at 65,536 and extend to over 4 billion. ASNs above 65,535 are sometimes called "4-byte" or "asplain". All modern BGP routers support both. The lookup handles either notation: AS15169, AS3320001, dot notation like AS65536.0, all valid.
Five RIRs and how ASNs get allocated
IANA delegates ASN blocks to five Regional Internet Registries:
- ARIN (North America, Caribbean, parts of the Atlantic)
- RIPE NCC (Europe, Middle East, central Asia)
- APNIC (Asia-Pacific)
- LACNIC (Latin America and Caribbean)
- AFRINIC (Africa)
Each RIR allocates ASNs to operators in its region according to its own policies. To get an ASN you typically need to be operating a multihomed network (peering with at least two upstreams) and meet a minimum size threshold. Costs range from a few hundred euros (RIPE) to a few thousand dollars (ARIN) per year.
The AS_PATH and how routes are chosen
When AS1 announces a prefix to AS2, the announcement carries AS1's number in the AS_PATH attribute. As AS2 re-announces to AS3, it prepends its own number. By the time the announcement crosses 10 networks, the AS_PATH might look like AS10 AS9 AS8 ... AS1. Routers prefer shorter AS_PATHs by default, so traffic naturally follows the most direct sequence of networks. The path you see in a traceroute is roughly the inverse: each hop's IP belongs to one AS, and reading the trace top to bottom shows you which networks the packet crosses.
Tier 1, Tier 2, Tier 3 networks
The "tier" terminology is informal but useful. Tier 1 networks reach the whole internet through peering alone, paying nobody for transit. There are perhaps a dozen, including AS174 (Cogent), AS2914 (NTT), AS3356 (Lumen, ex Level 3), AS1299 (Arelion, ex Telia Carrier), AS6453 (Tata), AS6762 (Sparkle), AS6939 (Hurricane Electric is often called Tier 1.5 because some peers refuse to settle-free with them). Tier 2 pays some transit but also peers heavily. Tier 3 pays for all its connectivity. Big CDNs (Cloudflare AS13335, Google AS15169, Akamai AS20940, Meta AS32934, Amazon AS16509) are not "tier 1" in the classical sense, they are content networks that peer widely.
Famous and notable ASNs to know
- AS15169 Google. The biggest content network, ~12% of all internet traffic.
- AS13335 Cloudflare. CDN and DDoS protection in front of millions of sites.
- AS16509 Amazon AWS. Hyperscaler cloud and CDN.
- AS32934 Meta. Facebook, Instagram, WhatsApp infrastructure.
- AS3356 Lumen Technologies (Level 3 legacy). One of the oldest tier-1s.
- AS174 Cogent. Tier 1, famous for the 2008 IPv6 peering war with Hurricane Electric.
- AS1299 Arelion (formerly Telia Carrier). European tier-1 backbone.
- AS2914 NTT Global. Japanese tier-1 with strong APAC reach.
- AS6939 Hurricane Electric. Largest IPv6 network by peering count.
- AS7018 AT&T. Big US incumbent ISP.
- AS3320 Deutsche Telekom. Largest European telco.
- AS24940 Hetzner. Popular German bare-metal and cloud host.
- AS14061 DigitalOcean. Developer-friendly cloud host.
Private ASNs and reserved ranges
Two ranges are reserved for private use, similar to private IP ranges. 64,512 to 65,534 for 16-bit, and 4,200,000,000 to 4,294,967,294 for 32-bit. They never appear in the public BGP table. Enterprises use them inside their own networks for internal BGP (iBGP) without consuming public ASN space. AS0 means "no AS" and is sometimes used in route filtering. AS23456 ("AS_TRANS") was a transition placeholder for 32-bit ASNs back when not all routers supported them, now mostly historical.
IRR, route objects and prefix filters
Networks publish their intent to announce specific prefixes in IRR (Internet Routing Registry) databases. The route objects in IRR are how upstream providers know what to allow from a customer's BGP session. Without a matching route object, an announcement gets filtered before it leaves the customer's edge. RPKI is the cryptographic upgrade to this. IRR is essentially a trust-based registry, RPKI is signed. Most networks still rely on IRR for filtering but combine it with RPKI validation.
Route collectors and where BGP data comes from
The global BGP table is not a single source of truth. Different routers see slightly different views depending on who they peer with. RIPE RIS and RouteViews are public projects that collect BGP feeds from hundreds of cooperating networks and publish the merged table. Looking glass servers (like Hurricane Electric's bgp.he.net) provide interactive lookups. The result page combines route-collector data with the RIR registry record for the AS.
How much does the BGP table grow each year
About 5% per year recently, currently around 980,000 IPv4 routes and 195,000 IPv6 routes. The growth comes from prefix disaggregation (operators splitting larger blocks into smaller announcements for traffic engineering) and from new IPv6 deployments. Some legacy networks have memory limits on older routers and have to upgrade hardware to keep the full table. Default-free zone (DFZ) carriers see the full table, edge ISPs often take a partial feed.
Customer cone and the AS rank
A network's customer cone is itself plus all the ASNs that depend on it for transit, plus all of theirs, recursively. The bigger the cone, the more of the internet sits "behind" you. CAIDA publishes AS Rank derived from the customer cone size. Lumen, NTT and Telia score highly because thousands of smaller networks pay them for transit. Cloudflare's customer cone is essentially zero (they sell services, not transit) but their peering footprint is huge.
Frequently Asked Questions
What is an ASN lookup?
An ASN lookup queries the registry record for a given Autonomous System Number. It returns the operator, country, RIR, registration date, and the IPv4 and IPv6 prefixes the AS announces in BGP, along with its peers and upstreams.
What is the difference between an ASN and an IP address?
An IP address is a single endpoint. An ASN is the network that owns and announces a range of IP addresses in global BGP routing. Every IP on the public internet sits inside some ASN's prefix.
How do I find the ASN behind a specific IP?
Use the IP lookup. The result page lists the announcing ASN. From there you can drill into the full prefix list and peering data.
What are 16-bit vs 32-bit ASNs?
Originally ASNs were 16-bit (1 to 65535). When that pool ran low, IANA introduced 32-bit ASNs in 2007 (RFC 4893). Numbers above 65535 are 4-byte. The lookup handles both formats.
Is this tool free?
Yes. Unlimited lookups, no registration, no API key needed for the web interface. The free API at api.ipwhois.net is available for programmatic access.
What is BGP and why does it matter?
Border Gateway Protocol is the routing protocol that lets ASNs exchange route announcements. It is the backbone of the global internet. The lookup shows which ASN currently announces a given prefix, who its peers are, and which upstream transit providers it pays.
Can I see all prefixes announced by an ASN?
Yes. The result page lists all IPv4 and IPv6 prefixes the ASN announces in BGP, with size, country and registry.
What is RPKI?
Resource Public Key Infrastructure. ASNs publish signed authorisations (ROAs) declaring which prefixes they are entitled to announce. Routers that validate against RPKI drop announcements that fail the check, preventing some classes of BGP hijack. Adoption is around 42% of routes today.