Network

What is ASN?

An Autonomous System Number identifies a network that announces its own IP address ranges to the rest of the internet through BGP. Every routable address sits inside a prefix announced by some AS, so the ASN behind an IP tells you who operates it - a mobile carrier, a hosting provider, a home ISP or a corporate network.

Regional internet registries such as RIPE NCC, ARIN and APNIC hand AS numbers to any organisation that runs its own routing policy and peers with other networks. The original field was 16 bits, which capped the space at 65,536 numbers, and 32-bit ASNs were added once allocations ran short. That is why you see short numbers like AS15169 alongside numbers well above one hundred thousand. The block from 64512 to 65534 is reserved for private use inside an organisation and never appears in the public routing table.

To find the AS behind an address you can query WHOIS or RDAP at the registry, use Team Cymru's IP-to-ASN service, or read an offline dataset such as MaxMind's GeoLite2 ASN database. They all work the same way underneath. Each maps the address to the most specific prefix currently announced in BGP and returns that prefix's origin AS. Announcements change hands, so an address that traced back to one operator last quarter can sit behind a different one today, which is why cached ASN data goes stale.

Anti-fraud and bot-detection vendors rarely score raw IP addresses on their own. They score the AS and the type assigned to it - hosting, ISP, mobile, education, business - because that classification stays stable while individual addresses churn through customers. A browser session arriving from a hosting AS carries an obvious signal that a server is driving it, and blocking that network costs the site almost no human visitors. Mobile carrier space behaves differently, since CGNAT packs thousands of real subscribers behind each address and a blanket block hits paying customers.

An ASN is not a location. One carrier can announce prefixes across many countries, MVNOs ride on the host operator's AS, and anycast puts the same prefix in several places at once, so geolocation databases and ASN records often disagree about where a user sits. The lookup is still the fastest audit of a proxy provider's claims. Pull a handful of sample exits and check whether they land on carrier space or on a hosting AS. The answer takes a minute and does not depend on anything the provider says about itself.

Where you meet it

You reach for an ASN lookup when a target starts blocking and you need to know whether the problem is one exit or the whole network behind it. Check the operator and the network type, then compare against an IP that still works. Buyers run the same lookup before committing budget, pulling several sample endpoints from a provider to see whether the origin AS belongs to a carrier or to a datacenter.

Common questions

How do I find the ASN of an IP address?

Run a WHOIS or RDAP query against the address and read the origin AS field, or use Team Cymru's IP-to-ASN lookup service. For bulk work, load an offline mapping such as GeoLite2 ASN and match addresses against announced prefixes locally, which avoids rate limits on public WHOIS servers.

Does a mobile carrier ASN prove an IP is a real mobile proxy?

No. The ASN tells you who announces the prefix, and carriers also allocate space to fixed-wireless, business and hosting products. Treat it as one signal, then confirm with reverse DNS, whether the address behaves like CGNAT-shared subscriber space, and how the target site responds in practice.

Why do sites treat hosting ASNs differently from ISP ASNs?

Address space on a hosting AS is sold to servers, so a full browser session originating there is already anomalous, and blocking the network costs the site few real visitors. Consumer ISP and mobile carrier space carries paying customers, so operators there apply challenges rather than outright blocks.

Related terms

Carrier
A carrier, or mobile network operator, is the company that owns the radio network and core infrastructure a phone connects to, such as AT&T, Vodafone or Orange. It assigns each device an address from its own IP ranges and ASN, which is what makes traffic from a mobile proxy read as an ordinary subscriber to the sites receiving it.
IP Reputation
IP reputation is the trust score a website, mail server or anti-bot system assigns to an IP address based on the traffic it has produced before. Spam complaints, request patterns, past abuse reports, hosting type and ASN ownership all feed that score. A poor one earns CAPTCHAs where a good one passes, or a refusal with no explanation.
IP Address
An IP address is the numeric identifier a device presents on an IP network so packets can be routed to and from it. IPv4 writes 32 bits as four decimal octets, such as 203.0.113.7. IPv6 writes 128 bits as hexadecimal groups, such as 2001:db8::1. Every request a server logs is tied to one.
IPv4
IPv4 is version 4 of the Internet Protocol, addressing hosts with 32 bits written as four dot-separated octets, which gives roughly 4.3 billion addresses. The top-level pool ran dry in 2011, pushing providers toward private addressing, carrier-grade NAT and a paid transfer market. Nearly all proxy traffic still runs over IPv4 because that is what target sites accept.
IPv6
IPv6 is version 6 of the Internet Protocol, addressing hosts with 128 bits written as eight hexadecimal groups, such as 2001:db8::1. The space is large enough that carriers assign each subscriber a whole subnet, usually a /64 or wider, so reputation systems score prefixes instead of single addresses. Adoption is broad but far from universal.
CGNAT
Carrier-grade NAT is address translation performed by an ISP rather than in the customer's router, putting many subscribers behind one public IPv4 address. Mobile networks depend on it. Each customer holds a private or shared-range address, usually from 100.64.0.0/10, and the carrier rewrites the source address and port on every outbound flow.
NAT
Network Address Translation rewrites the address and port fields of packets as they cross a router, letting many private hosts share one public IP. The router keeps a translation table, mapping each outbound flow to a free public port and reversing the rewrite on replies. Home routers do this for a household; carriers do it for whole regions.
DNS
The Domain Name System turns a hostname such as example.com into the IP addresses a client can connect to. A stub resolver on the device asks a recursive resolver, which walks the root, TLD and authoritative name servers, then caches the answer for the record's TTL. Queries travel on port 53, or over TLS or HTTPS when encrypted.

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