What Is an ASN? Autonomous System Numbers Explained
Every major network on the internet has an ASN — a unique number that identifies it for routing purposes. You see ASNs on IP lookup tools, in BGP routing tables, and in fraud-detection systems. Here's what they actually are, why they matter, and how to use them.
Quick answer
An ASN (Autonomous System Number) is a unique identifier assigned to a network on the internet — like ISPs, cloud providers, and major companies. Examples: Google is AS15169, Cloudflare is AS13335, Comcast is AS7922. ASNs power BGP routing and are used to identify network operators for traffic engineering, security, and IP intelligence.
What is an ASN?
An Autonomous System (AS) is a network or group of networks under a single administrative control with a unified routing policy. Every AS gets a unique number — the ASN — assigned by a regional internet registry (ARIN, RIPE, APNIC, LACNIC, or AFRINIC).
Think of the internet as a vast collection of independent networks. Comcast operates one big network. Google operates another. Cloudflare operates another. Each of these networks is its own "autonomous system" — they decide how to route traffic internally, but they need a way to communicate routing information with each other across the public internet.
That's where ASNs come in. Each network gets a unique ASN that identifies it globally. When networks exchange routing information via BGP (the protocol that makes inter-network routing work), they identify themselves by their ASN.
ASN format and numbering
ASNs are 32-bit integers, written either as a single number or with the "AS" prefix:
AS15169— GoogleAS13335— CloudflareAS7922— ComcastAS3356— Lumen (formerly Level 3)
16-bit vs 32-bit ASNs
The original ASN format was 16-bit, allowing only 65,536 possible numbers (0 to 65,535). By 2007, the internet had grown too large and these were running out. The IETF introduced 32-bit ASNs (RFC 6793), expanding the space to over 4 billion possible numbers.
Most well-known networks still have low (16-bit) ASNs because they got theirs early. Newer ASNs assigned today are typically 32-bit (above 65,535) and look like AS213045 or AS400321.
Reserved ASNs
Some ASN ranges are reserved for special use:
0— reserved (RFC 7607)23456— reserved for AS_TRANS, used when 32-bit ASNs communicate with 16-bit-only systems64512–65534— private ASNs (similar to private IPs, used inside organizations)65535— reserved4200000000–4294967294— private 32-bit ASNs4294967295— reserved
How ASNs work with BGP
BGP (Border Gateway Protocol) is the routing protocol that makes the internet work as a whole. ASNs are the identifiers BGP uses to communicate between networks.
Here's a simplified view of how it works:
- Each AS announces which IP ranges it owns via BGP. For example, Google's AS15169 announces ranges like
142.250.0.0/15. - BGP routers receive these announcements from peers (other ASes they're connected to).
- Each router builds a routing table mapping IP ranges to the path of ASNs needed to reach them.
- When traffic arrives destined for an IP, the router looks up the best path through neighboring ASes.
- The traffic hops from AS to AS until it reaches its destination network, where internal routing takes over.
This is essentially how packets traverse the internet. Your ISP's AS hands off traffic to a peer or transit provider, which hands it off to another, until it reaches the destination's AS.
BGP fragility: Because BGP relies on networks honestly announcing what IP ranges they own, BGP "hijacks" are possible — an AS deliberately or accidentally claiming IP ranges it doesn't own. This has caused several major internet outages, including the famous 2008 Pakistan/YouTube incident where Pakistan accidentally took YouTube offline globally.
Famous ASNs you'd recognize
If you do any IP investigation work, you'll quickly recognize these ASNs:
Cloud providers
AS15169— GoogleAS16509— Amazon AWSAS8075— Microsoft AzureAS13335— CloudflareAS14061— DigitalOceanAS63949— Akamai/LinodeAS20473— VultrAS24940— Hetzner
Major ISPs (US)
AS7922— Comcast CableAS7018— AT&T ServicesAS701— Verizon BusinessAS22773— Cox CommunicationsAS20115— Charter (Spectrum)AS209— CenturyLink/LumenAS21928— T-Mobile USA
Major ISPs (international)
AS3320— Deutsche Telekom (Germany)AS5089— Virgin Media (UK)AS4134— China TelecomAS9498— Bharti Airtel (India)
Backbone providers
AS3356— Lumen (formerly Level 3) — carries massive global transitAS1299— Arelion (formerly Telia)AS6453— TATA CommunicationsAS2914— NTT Communications
ASN vs. IP address
| ASN | IP Address | |
|---|---|---|
| Identifies | A network operator | A device or endpoint |
| Examples | AS15169, AS13335 | 8.8.8.8, 1.1.1.1 |
| How many exist | ~110,000 active globally | ~4.3 billion IPv4 + 340 undecillion IPv6 |
| Used by | BGP routing between networks | Routing to specific devices |
| One owner has | Usually one ASN | Thousands to millions of IPs |
| Visible to websites | Yes (via IP lookup) | Yes (it's how they reach you) |
| Changes how often | Rarely — stable for years | Often — dynamic IPs rotate frequently |
An ASN is the bigger umbrella. Your specific IP address belongs to a block of IPs owned by an AS. When you look up an IP, the ASN tells you which network it's part of.
Why ASN matters for security and intelligence
ASNs reveal whether traffic is coming from a residential ISP, a cloud provider, or a hosting company — which has major implications for security, fraud detection, and IP intelligence.
Fraud detection
Online fraud usually originates from data centers or VPN providers, not residential connections. When a "user" appears to be logging in from AS16509 (Amazon AWS) or AS14061 (DigitalOcean), fraud systems flag it — real people don't browse from AWS. Same goes for known VPN provider ASNs.
Bot detection
Web scraping bots, credential stuffers, and ad-fraud bots overwhelmingly come from cloud ASNs. Cloudflare, Imperva, and other CDN/security companies maintain ASN-based blocklists for this reason.
Ad targeting
Advertisers segment by ASN to identify users on premium ISPs (often correlated with income) or specific corporate networks.
Geographic routing
CDNs use ASN data to decide which edge servers serve which users for optimal latency.
BGP monitoring
Network operators monitor ASN announcements globally to detect route hijacking, prefix leaks, and DDoS amplification attacks.
OSINT and investigation
Journalists, researchers, and law enforcement use ASN data to identify network operators tied to suspicious activity. Tools like Shodan, Censys, and Hurricane Electric's BGP Toolkit are built around ASN intelligence.
How to find an ASN
There are several ways to look up ASNs:
1. Our IP Lookup tool (easiest)
Visit our IP Lookup tool and enter any IP. The ASN appears in the KPI card showing the network operator name. Quickest way to identify the AS behind any IP.
2. Visit scanmyipaddress.com
For your own ASN, just visit the homepage — your ASN is shown with your IP and ISP details.
3. Command line (whois)
On Mac/Linux, install whois if not already present, then run:
whois -h whois.cymru.com " -v 8.8.8.8"Returns the ASN, owner, country, and IP range. Or run a generic whois on the IP:
whois 8.8.8.84. BGP Toolkit (Hurricane Electric)
bgp.he.net is the industry-standard free tool. Enter any ASN, IP, or domain to see routing details, IP prefixes, peers, and BGP relationships.
5. Programmatic APIs
If you're building software that needs ASN data, the major IP intelligence APIs all provide it:
- ip-api.com (free, what our IP Lookup uses)
- ipinfo.io
- MaxMind GeoLite2 (downloadable database)
- Team Cymru's IP-to-ASN service (DNS-based, free for low volume)
Frequently asked questions
What is an ASN?
An ASN (Autonomous System Number) is a unique identifier for a network on the internet. Large networks like ISPs, cloud providers, and major companies each have their own ASN, which is used for routing traffic between them. For example, Google's ASN is AS15169, Cloudflare's is AS13335, and Comcast's is AS7922.
How do I find my ASN?
The easiest way: visit scanmyipaddress.com — your ASN is displayed with your IP details. You can also use our IP Lookup tool with any IP to see its associated ASN. Your ASN belongs to your ISP, not you personally — you share it with every other customer on the same network.
Is an ASN the same as an IP address?
No. An IP address identifies a single device or network endpoint. An ASN identifies an entire network operator (like Comcast or Google). One ASN typically owns thousands or millions of IP addresses. Think of ASN as the network identity and IP as the street address within that network.
Why does ASN matter?
ASNs power BGP, the routing protocol that makes the internet work. They're also used for IP intelligence: identifying whether an IP belongs to a cloud provider (potentially a VPN/bot), a residential ISP (real users), or a hosting company. Fraud detection, ad targeting, and security tools rely heavily on ASN data.
How do I get my own ASN?
You apply through a regional internet registry (ARIN in North America, RIPE in Europe, etc.). Requirements vary but generally you need: a multihomed network (connections to multiple upstream providers), your own IP prefix to announce, a unique routing policy, and a few hundred dollars in annual fees. Most organizations don't need their own ASN.
Can two ASNs share an IP range?
Not legitimately. Each IP block should be announced by only one ASN at a time. When two ASNs announce the same block (called a "BGP hijack"), routing becomes inconsistent and users may reach the wrong network. This has caused major outages, including the 2017 Cloudflare/Verizon Pixar incident.
Do I have my own ASN as a regular user?
No. ASNs belong to networks, not individual users. You share an ASN with every other customer of your ISP — potentially millions of people. Only large organizations that operate their own multihomed networks (ISPs, big cloud providers, large enterprises, universities) need their own ASN.