What is an ASN? The number behind every IP

An ASN, short for autonomous system number, is the number that identifies one network in internet routing. A network that wants its addresses reachable from the rest of the internet announces them under its AS number, so an address you can reach has a network behind it, and that network has a number. Not every allocated address is announced, though. The BGP table report at bgp.potaroo.net, in the version generated on November 10, 2026, shows 72.78% of IPv4 space advertised. The rest has no ASN to find.

Our ASN lookup answers the question for one address. This page covers what the answer means: who issues the numbers, what a network does with one, and where a lookup stops being useful. The lookups below can be repeated from a terminal.

Who issues the numbers, and what BGP does with them

IANA keeps the master list. It allocates AS numbers to the five Regional Internet Registries, AFRINIC, APNIC, ARIN, LACNIC and RIPE NCC, and those registries assign them to network operators in their own regions, under their own policies. That is from the IANA registry, which says it was last updated on 2026-06-01. When a lookup says "registry", it means one of those five.

Once a network has a number, it runs BGP, the Border Gateway Protocol, with the networks it connects to. Two neighbours tell each other which blocks of addresses they can reach. A block is written as a prefix, such as 75.75.0.0/17. RFC 4271, the BGP specification, has every route carry the list of AS numbers it has passed through, which is how a router sees the path and avoids loops. The network that first put the route into BGP is its origin, and the origin is the network a lookup reports. To announce a prefix is to originate that route.

RFC 1930, from March 1996, defines the unit: "An AS is a connected group of one or more IP prefixes run by one or more network operators which has a SINGLE and CLEARLY DEFINED routing policy." The capitals are the RFC's. The RFC then explains that routing policy means "how routing decisions are made in the Internet today", and that "it is the exchange of routing information between ASes that is subject to routing policies." Put plainly, it is a choice about which routes a network accepts from its neighbours and which it passes on.

One range is not issued to anyone in particular. RFC 6996 reserves 64512 to 65534 and 4200000000 to 4294967294 for private use, so a network can run BGP internally without a registry number. The RFC says these numbers must be removed from AS paths before a route goes out to the global internet, so they are not meant to appear in the global table.

16-bit and 32-bit numbers, and how many there are

AS numbers were 16 bits wide. The base BGP specification encoded them as a "two-octet entity", in the words of RFC 6793. The first standard for four-octet numbers was RFC 4893, published in May 2007. RFC 6793 replaced it in December 2012 and is the one in force. That is why a number can run as high as 4,294,967,295, and why our lookup accepts anything up to that.

On November 10, 2026 at 16:01 UTC, the same BGP table report counted 79,884 unique AS numbers. That is the count seen in routing that day, not the count registered, and the page changes daily.

One company can hold several numbers. Our list of networks has three Comcast numbers: AS7922, AS33491 and AS33651. Operators grow by buying other networks, and each one can keep the number it had, so AS7922 alone does not mean "all of Comcast." Our page for it lists 1,528 IPv4 ranges holding 42,195,200 addresses in the dataset dated 2026-11-01. AS7018, which the dataset names ATT-INTERNET4, has a page of its own.

Three addresses to look up

The addresses are 8.8.8.8 and 1.1.1.1, two public DNS resolvers, and 75.75.75.75, which sits in Comcast's range. The lookups below were run on the evening of November 10, 2026, US Eastern.

Our lookup takes an IPv4 address or an AS number and answers from the public iptoasn.com dataset, which the page dates. The API behind the page returns the same answer:

for q in 8.8.8.8 1.1.1.1 75.75.75.75; do
  curl -s "https://resifactory.net/api/tools/asn?q=$q" \
    | jq -c '{ip, asn, name, country, hosting, datasetDate}'
done

{"ip":"8.8.8.8","asn":15169,"name":"GOOGLE","country":"US","hosting":true,"datasetDate":"2026-11-01"}
{"ip":"1.1.1.1","asn":13335,"name":"CLOUDFLARENET","country":"US","hosting":true,"datasetDate":"2026-11-01"}
{"ip":"75.75.75.75","asn":7922,"name":"COMCAST-7922","country":"US","hosting":null,"datasetDate":"2026-11-01"}

The datasetDate is the day the data was read, and we refresh it about once a month. An address that changed hands after the first of November would not show yet.

A second source gives you a check that doesn't depend on ours. whois can ask Team Cymru's IP to ASN service, which maps an address to its BGP origin ASN from BGP feeds that its page says are refreshed every four hours:

whois -h whois.cymru.com " -v 8.8.8.8"

AS      | IP               | BGP Prefix          | CC | Registry | Allocated  | AS Name
15169   | 8.8.8.8          | 8.8.8.0/24          | US | arin     | 2023-12-28 | GOOGLE - Google LLC, US
13335   | 1.1.1.1          | 1.1.1.0/24          | AU | apnic    | 2011-08-11 | CLOUDFLARENET - Cloudflare, Inc., US
7922    | 75.75.75.75      | 75.75.0.0/17        | US | arin     | 2006-07-07 | COMCAST-7922 - Comcast Cable Communications, LLC, US

That is three runs with the address changed, and the header printed once. The numbers are the same as above: 15169, 13335, 7922. The two lookups draw on different data sources and agree on all three, though both reflect the global BGP table, so they are not independent evidence of the table itself.

The registrant and the announcer are different records

RDAP is the registries' lookup service. RFC 7480 calls it a successor protocol to the very old WHOIS protocol. It returns JSON, which is long, so this loop keeps four fields: the address range, the network name, and the name of the entity whose role is registrant.

for a in 8.8.8.8 1.1.1.1 75.75.75.75; do
  curl -sL "https://rdap.org/ip/$a" | jq -r --arg a "$a" '[$a, (.startAddress+"-"+.endAddress), .name, (.entities[]|select(.roles|index("registrant"))|.vcardArray[1][]|select(.[0]=="fn")|.[3])]|join(" | ")'
done

8.8.8.8 | 8.8.8.0-8.8.8.255 | GOGL | Google LLC
1.1.1.1 | 1.1.1.0-1.1.1.255 | APNIC-LABS | APNIC Research and Development
75.75.75.75 | 75.75.72.0-75.75.79.255 | COMCAST-47 | Comcast Cable Communications Holdings, Inc

The middle row is the one to read. The registrant of 1.1.1.0/24 is APNIC Research and Development, and the network name is APNIC-LABS. The Cymru line for the same address puts AU in its country column, which is the registry's. The network that announces the block is Cloudflare's, AS13335, and the record says so in a remark: "Routed globally by AS13335/Cloudflare."

So there are two records. The registry names who holds a block of addresses, and routing names who announces it. An ASN is the routing answer. It is the one that decides where packets go.

The third row has a smaller wrinkle. RDAP lists 2,048 addresses for 75.75.75.75 (75.75.72.0 to 75.75.79.255), while the announced prefix is 75.75.0.0/17, which contains it. A holder can carve its space into small records and still announce one big prefix, so two ranges for one address do not mean the lookup is wrong.

An AS number has a record of its own, too. curl -sL https://rdap.org/autnum/7922 returns the handle AS7922, the name COMCAST-7922, a registrant of Comcast Cable Communications, LLC, and a registration event dated 1997-02-14.

What an ASN can't tell you

It does not tell you where the address is. The country in a lookup is the country of the registration. For 1.1.1.1 the allocation says Australia and the operator's name says the US, and neither says where the machine answering sits. Team Cymru says as much of itself: "This is not a geolocation service: registry country codes often differ from where an address is actually used." Locating an address is a different job, done by other databases.

It does not tell you who is using the address. A broadband network announces addresses that homes use and addresses that its business customers use, under one number. The number names the carrier, not the customer on the line.

It does not always tell you what kind of network it is, either. GOOGLE and CLOUDFLARENET are on our list of hosting and cloud networks, and our tool says so with "hosting": true. Comcast came back null, not false, because that field is true only when a number is on our list. Absence from a list says nothing about what a network is, so null means "we haven't labelled it" and does not mean residential.

One limit is ours. The dataset our lookup reads is IPv4 only, so for an IPv6 address use whois or RDAP, which handle both.

Why it matters when you buy proxies

A proxy gives you an exit address, and anyone on the other side can run the same lookup on it. The announcing ASN is the quickest way to see what you are getting: a broadband provider, a mobile carrier or a hosting company. If you paid for a residential exit and the lookup returns AS15169, you did not get one. We wrote down what these lookups can and can't prove about a vendor's claims in Does your proxy vendor run a network or rent one?.

On our side, every pool accepts a network as a target: Essential and Premium Residential, and Mobile. A request can ask for one network by AS number with the asn option, written AS7922 or 7922, as the API reference describes it. The docs add that the option is not applied on geo-forced pools. If no exit on that network is available, the request fails upstream; it does not fall back to a different network. We don't publish which networks are available at a given time, so the list on our lookup pages tells you that a network exists, not that it is in the pool. Details are on the residential proxies page.

Take ten exits from the vendor you are testing and look each one up in the ASN lookup. If any of them comes back as a hosting network and you paid for residential or mobile, send it to support. The hosting list is ours and can miss a network, so read the name as well as the flag.