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What is an ASN (Autonomous System Number)? How to tell which network an IP belongs to

When you look up an IP you often see a label like “AS13335” or “AS15169”. That is an ASN, an Autonomous System Number: the number of the network the IP sits in. An ASN tells you which network is announcing the address range in the global routing table right now. It doesn’t tell you whether the IP is home broadband or a datacenter, and it isn’t the country the IP is in. This guide covers what an ASN is, who hands them out, how an IP maps to one, and how to look it up on IP Judge.

At a glance: from an IP to its networkUsing 1.1.1.1 as the example: the prefix covering it in the routing table is 1.1.1.0/24, that prefix is announced by AS13335, and AS13335 is held by CLOUDFLARENET - Cloudflare, Inc., registered in the US with ARIN. An ASN tells you who announces the range, not the IP's type or the country the IP is in. At a glance: from IP to network The IP you check 1.1.1.1 Prefix that covers it 1.1.1.0/24 Routing Network announcing the prefix AS13335 Routing Holder of the ASN CLOUDFLARENET - Cloudflare, Inc. The ASN itself is registered in the US (ARIN) Registry An ASN says who announces the range Not the IP’s type, and not where the IP is Example data from RIPEstat, queried 2026-10-01.
To find an IP’s ASN, find the prefix that covers it in the routing table, then see which network announces that prefix.

What an autonomous system is

The internet is not one network but many independently run networks connected together. In routing terms, each of them is an autonomous system (AS).

RFC 1930 (BCP 6) defines an AS as a connected group of one or more IP prefixes, run by one or more network operators, with a single, clearly defined routing policy. Each AS has a globally unique number, the ASN, which identifies it when networks exchange routing information.

ISPs, cloud providers, content networks, large companies and universities usually have their own ASN, and one organisation can have several. On IP Judge’s page for AS15169 (Google), for example, the downstream list includes AS396982 (GOOGLE-CLOUD-PLATFORM), another Google network.

16-bit and 32-bit ASNs

ASNs started as 16-bit (2-byte) numbers, 0 to 65535. To avoid running out, RFC 6793 added BGP support for 32-bit (4-byte) ASNs, 0 to 4294967295. ARIN notes there is no longer a distinction between the two: all ASNs should be treated as 4-byte.

Some ranges are never assigned to a real network. From IANA’s AS number registry:

  • 64512–65534 and 4200000000–4294967294 are for private use and shouldn’t appear in public routing.
  • 64496–64511 and 65536–65551 are reserved for documentation and examples.
  • 23456 is AS_TRANS, a placeholder defined in RFC 6793 for older equipment that only understands 16-bit ASNs.

Who assigns ASNs

Numbers are handed down in layers. IANA runs the top of the pool and allocates ASNs to the five Regional Internet Registries (RIRs), which then allocate or assign them to network operators under their own regional policies. RFC 7020 describes the hierarchy: IANA, then the RIRs, then Local Internet Registries (LIRs) and other customers. IP addresses follow the same path.

The five RIRs and their regions (NRO overview):

  • AFRINIC: Africa
  • APNIC: parts of Asia and the Pacific
  • ARIN: North America and parts of the Caribbean
  • LACNIC: Latin America and parts of the Caribbean
  • RIPE NCC: Europe, the Middle East and parts of Asia

Getting an ASN has conditions. APNIC requires the network to be multihomed (connected to more than one upstream) or to need to interconnect with another AS; RIPE NCC’s current policy requires multihoming and a unique routing policy. RFC 1930 also says a single-homed site doesn’t need its own AS: its prefixes belong in its provider’s AS. That is why many home broadband and business lines show their ISP’s ASN.

Where AS numbers come fromIANA manages the top of the pool and hands ASNs and IP addresses to the five Regional Internet Registries: AFRINIC, APNIC, ARIN, LACNIC and RIPE NCC. The RIRs pass them on to LIRs, ISPs, cloud providers and companies under regional policy. ASNs come in 16-bit (0–65535) and 32-bit (0–4294967295) lengths, and all are now treated as 32-bit. Where AS numbers come from IANA Top of the pool: hands ASNs and addresses to RIRs Five Regional Internet Registries AFRINIC: Africa APNIC: parts of Asia and the Pacific ARIN: North America, parts of the Caribbean LACNIC: Latin America, parts of the Caribbean RIPE NCC: Europe, Middle East, parts of Asia LIRs, ISPs, clouds, companies Apply for ASNs and addresses under RIR policy Two ASN lengths 16-bit: 0–65535 32-bit: 0–4294967295 (all treated as 32-bit now) e.g. AS13335 sits in 13312–15359, a block IANA gave to ARIN.
Numbers flow down: IANA to the RIRs, the RIRs to network operators. The RIR in brackets on an ASN page is the one that issued the number.

How an IP maps to an ASN

Networks exchange routes with BGP. Under RFC 4271, BGP advertises destinations as IP prefixes (such as 1.1.1.0/24), and each advertisement carries the list of ASes it has passed through (the AS_PATH). The network that originates the route puts its own ASN there.

So “which ASN does this IP belong to” normally means: find the prefix in the global routing table that covers the IP, then see which ASN originates it.

You can see this data through RIPE NCC’s RIS, a platform that collects BGP data exchanged between networks. RIPEstat’s prefix overview endpoint returns whether a prefix is announced and by which ASN. A prefix can also be announced by more than one ASN at once, which RIPEstat calls multi-origin. To try it yourself:

curl "https://stat.ripe.net/data/prefix-overview/data.json?resource=1.1.1.1"

In the result, resource is the announced prefix covering the IP and asns lists the networks announcing it.

Registry and routing answer different questions

whois, and its newer replacement RDAP (RFC 9083), query the RIRs’ registration databases: who a range is registered to. Routing data answers a different question: who is announcing it now. They can be the same organisation, but don’t have to be.

Take 1.1.1.0/24. On 1 October 2026, APNIC’s RDAP record listed the registrant as APNIC Research and Development (network name APNIC-LABS, country AU). RIPEstat’s routing data showed the prefix announced by Cloudflare’s AS13335. And AS13335 itself falls in a block IANA gave to ARIN, with the US as its registered country.

Registry and routing answer different questionsThe same range, 1.1.1.0/24: the registry (RDAP / whois) shows it registered to APNIC Research and Development, country AU; the routing table (RIPEstat / RIS) shows it announced by AS13335, Cloudflare, Inc., an ASN registered in the US. The registrant and the announcer can differ, and an ASN's country is not the IP's country. Registry vs routing Same range: 1.1.1.0/24 Live example Registry RDAP / whois Who it’s registered to APNIC Research and Development Country: AU Routing table RIPEstat / RIS Who announces it now AS13335 Cloudflare, Inc. ASN country: US Registrant and announcer can differ And an ASN’s country isn’t the IP’s country Data: APNIC RDAP and RIPEstat, queried 2026-10-01.
The registry records who a range is registered to; the routing table records who announces it. They can be the same organisation, but don’t have to be.

Why the ASN matters when checking an AI account setup

ChatGPT, Claude and Gemini see your exit IP, and the ASN helps you tell what kind of network that exit is on: a cloud provider, a content network or an ISP. If your exit sits in AS13335 (Cloudflare) or a cloud provider’s ASN, for example, it is almost certainly not home broadband.

But an ASN is a clue, not a verdict:

  • One ASN can hold different kinds of addresses. An ISP’s ASN may include home broadband, business lines and ranges handed to datacenters; see residential vs datacenter vs ISP IPs.
  • The announcer isn’t always the user. 1.1.1.0/24 above is registered to one organisation and announced by another.
  • Network types are self-reported. PeeringDB’s network type is a category each network picks for itself (see the PeeringDB API docs). On 1 October 2026, AT&T (AS7018) and Verizon (AS701), both large US home broadband providers, were listed there as NSP (backbone / transit).

So IP Judge doesn’t classify a single IP by its ASN alone. It starts from the database’s operator type and datacenter / proxy flags for that range, then cross-checks reverse DNS and RDAP registry data. The ASN’s own type only fills in when the operator type is missing, and the PeeringDB type is shown but never used in the verdict. An IP judged to be a datacenter IP loses 25 points from its purity score and is flagged as more likely to get verification prompts. The full rules are on the method page.

An ASN’s country isn’t the IP’s country

An ASN has a registered country, but its addresses can be used anywhere. In the example above, AS13335 is registered in the US, 1.1.1.0/24 is registered in AU, and on 1 October 2026 IP Judge geolocated 1.1.1.1 to Australia. Large cloud providers work the same way: the ASN is registered where the company is based, while ranges in each region are often registered locally.

When people call an IP “native” or “broadcast”, they are comparing the range’s registered country with where the IP geolocates. Comparing the ASN’s country instead gets cloud ranges wrong. AI platforms likewise go by where your exit IP geolocates; see the country table for each region.

Looking it up on IP Judge

  1. Your own exit: run the check on the home page. The “Network” field shows the ASN and network name, and “Operator” shows who uses the range and what type it is. Click the ASN to open its page. A single IP has a page like /en/ip/1.1.1.1, with a trail of country › ASN › the /24 it sits in › IP (a /48 for IPv6). The ASN on IP pages comes from ipapi.is, with DB-IP Lite as a fallback.
  2. An ASN: open an address like /en/asn/AS13335 (no trailing slash). /en/asn/13335 redirects there too.
  3. A prefix: open an address like /en/prefix/1.1.1.0/24, written as network address / prefix length.
  4. Cloud providers: the cloud provider pages list the ASNs of common providers, and break down by region those that publish official IP lists.

What an ASN page shows, and where it comes from:

  • Holder name and the issuing RIR: RIPEstat’s AS overview (the name as registered in whois).
  • Registered country: from RIR statistics, via RIPEstat.
  • Number of announced IPv4 and IPv6 prefixes and IPv4 addresses: announcements RIPEstat saw over the last two weeks, counted only if at least 10 RIS peers saw them. Up to 500 IPv4 prefixes are listed.
  • Upstream and downstream neighbours: networks RIPEstat observed next to this one in AS paths.
  • Network type: what the network registered on PeeringDB, for reference only.
  • AI platforms: estimated from the ASN’s registered country; your exit IP’s location is what actually counts.
  • IPs checked on IP Judge: datacenter and proxy IPs are listed; residential and mobile IPs are only counted.

A prefix page shows which ASN (or ASNs) announces the range; if it is part of a larger announced prefix, it names that prefix. It also shows the allocating RIR, a reverse-DNS sample for /24s, and the provider and region when the range is on a cloud provider’s official list. Routing data refreshes daily.

FAQ

If the ASN belongs to a datacenter, is my IP definitely a datacenter IP?

Not necessarily. The network announcing a range may not be the one using it, and one network can hold ranges with different uses. For a single IP, go by the operator type, reverse DNS and registry data listed under “Evidence” on the result page.

Why do different sites show different ASNs for the same IP?

Different sources and update times. Some use live routing data, others their own periodically updated databases, and when a range changes hands or is announced by a new network, they catch up at different speeds. On IP Judge itself, IP pages take the ASN from ipapi.is (falling back to DB-IP Lite) while ASN and prefix pages use RIPEstat routing data, so the two can occasionally disagree.

Can one prefix be announced by several ASNs?

Yes. RIPEstat calls this multi-origin, and IP Judge’s prefix page lists every ASN announcing the range.

What does AS23456, or a number between 64512 and 65534, mean in routing data?

AS23456 is AS_TRANS, the placeholder RFC 6793 defines for older equipment that only understands 16-bit ASNs. 64512–65534 and 4200000000–4294967294 are private-use ranges meant only for internal use.

The ASN page says an AI platform is available. Does that mean it will work for me?

Not necessarily. That row is estimated from the ASN’s registered country; the platform looks at where your exit IP is and what type it is. Go by the exit check on the home page.

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