ASN Lookup
Free IP to ASN, BGP & IP Range Checker

What Is an ASN Lookup and Why Does It Matter?

An ASN lookup tells you which network operator controls an IP address. Every publicly routable address on the internet is announced by exactly one origin AS at any given moment, and an autonomous system lookup resolves that relationship in both directions — address to network, and network back to every address it holds.

An ASN — Autonomous System Number — is a unique identifier assigned to a network that operates under a single, consistent routing policy. Your ISP has one. So does Cloudflare (AS13335), Google (AS15169), and every hosting company you have ever rented a server from. The number is how the rest of the internet refers to that network when deciding where to send traffic.

Here is the part most explanations skip. An IP address tells you almost nothing on its own. 104.16.132.229 is just four numbers. Run an ASN lookup on it and you learn it belongs to Cloudflare, sits inside a prefix Cloudflare announces globally, and shares that network with millions of other sites behind the same reverse proxy. That single lookup changes how you interpret every log line containing that address.

Who Actually Uses ASN Lookups Day to Day

• Security analysts checking whether an attacking IP comes from a residential ISP or a bulletproof hosting network
• Network engineers planning peering or debugging why traffic takes a strange path
• Sysadmins who need the full IP range of a provider to build a firewall allowlist
• Fraud teams separating datacenter traffic from real consumer connections
• OSINT researchers mapping the infrastructure footprint of an organisation

Knowing what an ASN is only gets you halfway. The next section covers exactly what happens when you press the button above.

What Do Your ASN Lookup Results Actually Mean?

Each field in the result panel answers a different operational question. The AS number identifies the network, the holder name identifies the organisation, the prefix list defines its reachable footprint, and the peer lists show how it connects to everyone else. Read them together and you get a complete picture of an operator in about thirty seconds.

Reading the Prefix Count Correctly

Prefix count is not the same as network size. A network announcing 40 prefixes may control far more address space than one announcing 400, because a single /12 holds more addresses than four hundred /24 blocks. The tool shows both the prefix count and the total announced IPv4 address space so you can judge scale properly. If you want to work the maths yourself, our CIDR notation calculator converts any block into its exact host range.

Announced Versus Allocated

An AS can hold registry allocations it never advertises. This tool shows what is announced in BGP right now, because that is what is actually reachable and what your firewall cares about. Registry allocation data is a separate question, and our domain and IP WHOIS record lookup covers that side.

Once you can read a result, the obvious next question is how to get one without a browser at all.

Turning an ASN Into a Firewall Rule

Once you know which ASN a troublesome address belongs to, the next question is usually what to do about the rest of that network. You are typically trying to block a scraping operation, allowlist a cloud provider, or stop brute-force traffic from one hosting company.

The lookup above shows the announced prefix list for a single AS. When you need the complete set in a form you can paste, several AS numbers merged together, or adjacent blocks collapsed into fewer rules, that is what our ASN IP ranges downloader is built for. It exports to iptables, ipset, nftables, nginx, Cisco, MikroTik and a dozen other formats.

The ipset Commands You Actually Need

Blocking is a reaction. Understanding why these announcements exist at all takes one more layer of explanation.

RPKI and BGP Hijacking: How to Check Route Validity

RPKI — Resource Public Key Infrastructure, defined in RFC 6480 — lets an address holder cryptographically sign a statement declaring which AS is allowed to announce their prefix. That signed statement is a ROA (Route Origin Authorisation). When the tool above reports RPKI status, it is comparing the live announcement against the published ROA.

What a BGP Hijack Looks Like in Practice

A BGP hijack — when a network announces address space it does not control — can be accidental or deliberate. A fat-fingered prefix in a router config looks identical to an attack in the first ten minutes. The signature is the same either way: a prefix suddenly originating from an unexpected AS, usually with a more specific mask than the legitimate announcement, because routers always prefer the most specific route.

How Much of the Internet Is Actually Signed?

Adoption is now past the halfway mark but far from complete. The APNIC routing report for 2 May 2026 counted 624,805 IPv4 prefixes with a valid ROA, 423,253 with no ROA at all, and only 1,525 registering as invalid. Roughly six in ten routes are signed. The invalid count staying that low is the encouraging part — it means most operators who sign do it correctly.

If you operate address space, publishing a ROA is the single highest-value hour of security work available to you. Networks participating in MANRS increasingly drop RPKI-invalid routes outright, which means an unsigned prefix is not just unprotected — it is also easier to hijack silently.

Routing security is one use case. Threat intelligence is where most people first meet ASN data.

Common AS Numbers Reference: Cloud, CDN and Carrier Networks

A handful of AS numbers account for a large share of the traffic most operators see. Learning to recognise them saves a lookup every time one appears in a log. Every number below can be pasted straight into the tool above to pull its current prefix list.

Reserved and Private AS Number Ranges

• 64512 – 65534 — private 16-bit ASNs, per RFC 6996, never routed on the public internet
• 4200000000 – 4294967294 — private 32-bit ASNs, used inside large enterprise and datacenter fabrics
• AS0 — reserved by RFC 7607; a route with AS0 in the path must be discarded
• 23456 — AS_TRANS, a compatibility placeholder for legacy 16-bit-only routers

If a lookup returns an AS in a private range, you are almost certainly querying internal infrastructure rather than public routing data. Our explainer on public versus private addressing covers the same distinction at the IP layer.

One more thing changed the free ASN tooling landscape recently, and it is worth knowing about.

ASN vs IP Address vs CIDR Prefix: What Is the Difference?

These three terms describe different layers of the same system, and mixing them up leads to real configuration mistakes. An ASN identifies a network operator. A CIDR prefix identifies a block of addresses that operator announces. An IP address identifies a single interface inside that block.

The practical rule: block at the prefix level when you want precision, block at the ASN level when you want coverage and accept collateral damage. Never assume one AS equals one company — large organisations hold several AS numbers, and acquisitions leave the original numbers in place for years.

If subnet mathematics is where this gets confusing, our subnet mask and host range calculator and the plain-English subnet mask explainer both break it down step by step. For address-space history and why IPv6 matters here, see our breakdown of IPv4 exhaustion.