What Is an ASN (Autonomous System Number)?
An Autonomous System Number (ASN) is a unique identifier assigned to a collection of IP networks under a single administrative entity - Typically an ISP, a large company, a university, or a content delivery network. ASNs are used by the Border Gateway Protocol (BGP) to route traffic between autonomous systems across the internet. You can look up any ASN to see which organization controls it and which IP prefixes it announces.
ASN Format and Ranges
| Type | Range | Total Available | Status |
|---|
| 16-bit ASN (original) | 1 – 65,535 | 65,535 | Mostly allocated; some reserved |
| 32-bit ASN (extended) | 65,536 – 4,294,967,295 | ~4.29 billion | Actively assigned since 2007 |
| Private ASN (16-bit) | 64,512 – 65,534 | 1,023 | Not routed on public internet |
| Private ASN (32-bit) | 4,200,000,000 – 4,294,967,294 | 94,967,295 | Not routed on public internet |
| Reserved / Special | 0, 65,535, 4,294,967,295 | 3 | Reserved by IANA |
Well-Known ASNs
| ASN | Organization | Notes |
|---|
| AS15169 | Google LLC | Google's primary network |
| AS32934 | Meta Platforms (Facebook) | Facebook's global infrastructure |
| AS13335 | Cloudflare, Inc. | CDN and DNS provider (1.1.1.1) |
| AS16509 | Amazon.com, Inc. (AWS) | Amazon Web Services |
| AS8075 | Microsoft Corporation | Azure and Microsoft services |
| AS7018 | AT&T Services, Inc. | Major US ISP |
| AS3356 | Lumen Technologies (CenturyLink) | Tier-1 backbone provider |
Why ASNs Matter for Privacy and Security
- Every public IP address belongs to an AS - Your ISP's ASN is visible to every server you connect to. See yours via our IP lookup tool.
- Websites and security tools use ASN data to detect VPN, proxy, datacenter, and residential IP traffic.
- ASN-level blocking is common: a site may block all IPs in an ASN known for hosting VPN servers - Run a blacklist check to see if your IP's ASN is flagged.
- Security researchers use ASN data to attribute attacks to specific organizations or hosting providers.
- BGP hijacking - Where a rogue AS falsely announces ownership of another AS's IP blocks - Is a real threat to routing integrity.
