Acutis What Is an ASN?
What is an ASN, in plain English?
An ASN is an Autonomous System Number — a unique ID for a network that's run by a single organization. Every large network on the internet, whether it's an internet provider, a cloud company, or a big corporation, is assigned one or more ASNs. When you see something like AS15169, that's Google; AS13335 is Cloudflare; AS7922 is Comcast.
The short version: an ASN tells you who actually operates a chunk of the internet. If you have an IP address and want to know which company is really behind it, the ASN is the answer.
The internet is a network of networks
The word "internet" literally means "inter-network" — it isn't one giant network, it's tens of thousands of separate networks stitched together. Each of those independent pieces is called an autonomous system (AS): a collection of IP addresses under the unified control of one operator, following one routing policy.
Your home connection lives inside your ISP's autonomous system. A website you visit might live inside Amazon's or Cloudflare's. For your traffic to reach that site, these separate networks have to know how to hand packets off to one another — and that's where ASNs and routing come in. The ASN is the name tag each network wears so the others know who it's talking to.
How BGP announces routes between them
The networks of the internet advertise themselves to each other using the Border Gateway Protocol, or BGP. BGP is the conversation that happens between autonomous systems, and it works roughly like this:
- Each AS announces the IP blocks it owns — essentially saying, "to reach these addresses, send the traffic to me."
- Those announcements include the AS path: the chain of ASNs the traffic would cross to get there, written as a list of numbers.
- Neighboring networks pass the announcements along, so every router on the internet learns which AS to head toward for any given address.
This is why an ASN matters so much. Routing on the global internet is organized around autonomous systems, not individual machines. When you trace a path across the internet, you're really hopping from one ASN to the next until you land in the one that owns your destination.
What an ASN tells you
Once you know the ASN behind an IP address, you learn things the IP alone won't tell you:
- Who really operates the address. An IP might geolocate to a city, but the ASN tells you whether it belongs to an ISP, a cloud host like AWS, a VPN provider, or a specific company.
- Whether traffic is from a data center or a real user. Traffic from a hosting ASN often signals a bot, scraper, or server — useful for security and abuse decisions.
- The size and reach of a network. The IP blocks announced under an ASN show how much of the internet that operator controls.
- The routing relationships. The AS path reveals which networks a provider connects through to reach the rest of the world.
Look up any ASN or IP
Acutis Go's free tools resolve an ASN to its operator and announced IP blocks — or go the other way and find the ASN behind an address. No account needed.
Look up any ASN →Have an IP instead? Use the IP lookup tool to find the ASN that owns it.
IP-to-ASN lookups
The most common thing people do with ASNs is run an IP-to-ASN lookup: you start with an address you've seen in a log, an email header, or a firewall alert, and you ask "which autonomous system does this belong to?" The lookup maps the IP to the block that's announced in BGP and returns the owning ASN, the organization name, and usually the country and registry.
That single step turns a meaningless string of numbers into an answer you can act on — "this is Amazon's cloud," "this is a residential Comcast line," or "this is a hosting provider in another country." For anyone investigating suspicious traffic, sizing up a provider, or just satisfying curiosity about who runs an address, the ASN is the key that unlocks it.