What Is an Autonomous System (AS)?
The internet is not a single network owned by one organization. It is a collection of tens of thousands of independent networks — each called an Autonomous System (AS). An AS is a group of IP address ranges (prefixes) under the control of a single organization that has a unified routing policy for how traffic enters and exits its network.
Organizations that operate their own AS include: Internet Service Providers (ISPs like AT&T, Comcast, BT), large enterprises (banks, hospitals, universities), cloud providers (Amazon AWS, Google Cloud, Microsoft Azure), content delivery networks (Cloudflare, Akamai, Fastly), and government networks.
What Is an AS Number (ASN)?
Every Autonomous System is assigned a unique identifier called an AS Number (ASN). ASNs are 16-bit or 32-bit integers assigned by Regional Internet Registries (RIRs) — the same organizations that manage IP address allocation (ARIN in North America, RIPE NCC in Europe, APNIC in Asia-Pacific, LACNIC in Latin America, AFRINIC in Africa).
ASNs are written as plain integers prefixed with 'AS': for example, AS15169 belongs to Google, AS13335 belongs to Cloudflare, AS7922 belongs to Comcast. You can look up the ASN of any IP address using our IP Lookup tool — the 'AS' field in the results shows the ASN and registered organization name.
16-bit vs 32-bit ASNs
Originally, ASNs were 16-bit numbers (range: 1–65535). As the internet grew, 16-bit ASNs ran out, so 32-bit ASNs were introduced (range: 1–4,294,967,295). 32-bit ASNs are written in dot notation (e.g., AS131072 = AS2.0 in dot notation) or as plain integers. Most tools and databases show them as plain integers. The private ASN ranges (64512–65534 for 16-bit; 4200000000–4294967294 for 32-bit) are used internally within networks and not announced on the global internet.
How ASNs Work: BGP Routing
Autonomous Systems communicate with each other using BGP (Border Gateway Protocol) — the internet's routing protocol. Each AS announces its IP prefixes to neighboring ASes using BGP, saying: 'I own these IP blocks, send traffic for them to me.' Routers at the edges of each AS (called border routers) maintain BGP routing tables that map destination IP prefixes to the next AS hop.
When you load a webpage, your data travels from your ISP's AS through one or more intermediate transit ASes before reaching the destination server's AS. This chain of ASes is called an AS path. BGP selects the best path based on attributes like path length, local preference, and routing policies.
Types of Autonomous Systems
| Type | Description | Example |
|---|---|---|
| Tier 1 | Global backbone carriers with no upstream providers. They peer with each other to form the internet's core. | AT&T (AS7018), NTT (AS2914), Lumen (AS3356) |
| Tier 2 | Regional or national ISPs that buy transit from Tier 1s and peer with others at the same level. | Comcast (AS7922), BT (AS2856), Deutsche Telekom (AS3320) |
| Tier 3 | Local ISPs and end-user networks that buy all transit from upstream providers. | Small regional ISPs, enterprise networks |
| Content/Cloud | Large content providers with their own global networks that peer directly with ISPs to reduce transit costs. | Google (AS15169), Facebook (AS32934), Netflix (AS2906) |
How to Find the ASN of an IP Address
There are several ways to look up an ASN:
- Online tool: Use our IP Lookup tool — enter any IP and the ASN is shown in the results alongside the organization name.
- Terminal (whois): Run
whois -h whois.radb.net -- '-i origin AS15169'to see all prefixes announced by an ASN. - BGP looking glass: Many ISPs and IXPs run public looking glass servers where you can query live BGP routing tables.
- Team Cymru IP to ASN: A free DNS-based lookup:
dig +short 8.8.8.8.origin.asn.cymru.com TXT
Why ASNs Matter for Security and Privacy
ASNs appear in IP geolocation results because they tell you the operator of an IP — not just the geographic location. Knowing an IP belongs to AS13335 (Cloudflare) immediately tells you it's likely CDN or proxy infrastructure, not a residential user. This is why ASN data is critical for:
- Fraud detection: Cloud ASNs (AWS, DigitalOcean) create accounts at 100x the rate of residential ASNs — a key fraud signal.
- Bot detection: Most web scraping bots come from a small set of cloud provider ASNs.
- Content geo-restriction: Block entire cloud ASNs to prevent VPN users from bypassing geo-blocks.
- Threat intelligence: Track which ASNs are the sources of spam, DDoS, or brute-force attacks.
Notable ASNs
Some ASNs are globally well-known by network engineers:
- AS15169 — Google (Search, YouTube, Cloud, DNS 8.8.8.8)
- AS13335 — Cloudflare (CDN, 1.1.1.1 DNS, Workers)
- AS16509 — Amazon AWS
- AS8075 — Microsoft (Azure, Office 365, Teams)
- AS32934 — Meta (Facebook, Instagram, WhatsApp)
- AS2906 — Netflix
- AS7922 — Comcast (largest US cable ISP)
- AS3356 — Lumen/CenturyLink (major US Tier-1 backbone)
- AS2914 — NTT Communications (global Tier-1)
- AS1 — RIPE NCC (used for internal coordination)
BGP hygiene operators watch
IRR objects (RPSL in RADB or RIR databases), RPKI ROAs, and BGP AS-PATH filters should stay aligned with what you originate—stale route objects cause remote blackholes during migrations. IRR-only validation is weaker than RPKI; prefer ROV-enabled peers where possible. When investigating abuse, capture both the prefix and originating ASN from whois and BGP snapshots because anycast fronts can share addresses across ASNs.
For hijack response patterns see BGP explained and correlate ASN shifts with geolocation drift in your logs.