Introduction
When you look up an IP address, one of the fields often shown is ASN — Autonomous System Number. It's a technical networking term that most users have never heard of. Here's what it means and why it appears.
What Is an Autonomous System (AS)?
An Autonomous System (AS) is a large network or group of networks operated by a single organization — typically an ISP, university, government, large corporation, or content provider — that follows a common routing policy.
Examples of autonomous systems:
- Your home ISP (e.g., Comcast, BT, Airtel)
- A cloud provider (Amazon AWS, Google Cloud)
- A CDN (Cloudflare, Akamai)
- A university network
- A government network
What Is an ASN?
An ASN (Autonomous System Number) is a unique number assigned to each autonomous system by IANA (the Internet Assigned Numbers Authority) and distributed by the five Regional Internet Registries (RIRs).
Format: ASNs are written as "AS" followed by a number.
- Example:
AS15169is Google's ASN - Example:
AS7922is Comcast's ASN - ASNs can be 16-bit (1–65535) or 32-bit (for newer allocations)
Why Does My IP Lookup Show an ASN?
Your IP address belongs to a block of IPs allocated to a specific AS. When you do an IP lookup, the tool identifies which AS your IP belongs to and displays its ASN and organization name.
For most home users, the ASN will be your ISP's number and name.
Practical Uses of ASN Lookups
| Use Case | Why ASN Matters |
|---|---|
| Network security | Block all traffic from a specific AS (e.g., a known bad actor's hosting provider) |
| Geo-restriction | Some services block entire ASNs known to be used by VPN providers |
| Research | Understand the network topology and ownership of IP ranges |
| Fraud detection | IPs from hosting/data center ASNs have different risk profiles than residential ASNs |
How to Look Up an ASN
At what-is-my-ip.best, your ASN is shown in your IP lookup results. For deeper ASN research:
- bgpview.io — Full ASN details and IP prefix list
- ipinfo.io/AS##### — Details on any specific ASN
- team-cymru.com — ASN lookup and BGP data
Conclusion
An ASN is the identifier for the large network that owns your IP address — usually your ISP. It appears in IP lookup results because it's part of the public routing infrastructure that makes the internet work. For most users, it's just confirming which ISP you're on. For network engineers and security professionals, it's a critical routing and risk-assessment identifier.
Last updated: 2026 | Category: How IP Works