Try AS15169 (Google), AS8075 (Microsoft), AS3356 (a tier-1 backbone), or AS64512 to see a private-range example.
What is an ASN?
An ASN (Autonomous System Number) is a unique ID given to a network that runs itself: an internet provider, a big company, a university, a cloud platform. If a network decides on its own how traffic gets in and out, it gets an ASN so the rest of the internet can recognise it.
Think of the internet as a huge postal system made of thousands of independent delivery companies. Your street address (your IP) tells a letter where to end up. The ASN is more like the company badge on the van. It tells everyone which network is carrying your traffic at each step of the journey. AS numbers look like AS15169 or AS13335: the letters “AS” followed by a number.
How does an ASN actually work?
On its own, an ASN is just a label. It becomes useful because of a system called BGP, the way networks tell each other “I can reach these addresses, send that traffic to me.” Every announcement is tagged with the sender’s ASN, so a route across the world reads like a list of networks the traffic passed through: your ISP, maybe a national backbone, then the destination’s network. When you run a traceroute, each hop belongs to one of these autonomous systems, and an ASN lookup is how you turn those anonymous IPs into real company names.
Where are ASNs used?
You rarely see ASNs day to day, but they quietly power a lot of things:
- Finding who owns an IP. Security teams look up the ASN behind an address to see if traffic is coming from a home ISP, a data centre, or a known bad network.
- Troubleshooting slow connections. If a site is unreachable, the ASN in a traceroute shows exactly which network along the path is the problem.
- Deciding how traffic flows. Big networks use ASNs to choose the cheapest or fastest route, and to peer directly with each other.
- Blocking and allow-listing. Services can permit or deny whole networks by ASN, for example, treating cloud-hosted traffic differently from home users.
- Research and transparency. Journalists and researchers use ASNs to map outages, censorship, and who really hosts a service.
How do I find my ASN, or a website’s?
You don’t normally have your own ASN: the one behind your connection belongs to your internet provider. Here are the easy ways to find it:
- For your own connection: search “what is my IP” to get your public address, then paste that IP into an IP-to-ASN tool. It shows your provider’s AS number and name.
- For a website: find the site’s IP first (our DNS & WHOIS Lookup gives you the A record), then look that IP up to see whose network hosts it.
- If you already have the AS number: just type it into the box at the top of this page for the full registry record (operator, country, and abuse contacts).
- On the command line: a
traceroute(ortracerton Windows) to any site lists every network hop, and each hop maps to an ASN.
Networks you’ll see constantly
| ASN | Operator | Why you’ll meet it |
|---|---|---|
AS13335 |
Cloudflare | Half the web’s CDN, 1.1.1.1 resolver |
AS15169 |
Search, YouTube, 8.8.8.8 resolver | |
AS16509 |
Amazon (AWS) | A huge share of “the cloud” in traceroutes |
AS3356 |
Lumen / Level 3 | Classic tier-1 transit backbone |
AS2914 |
NTT | Tier-1 you’ll cross reaching Asia |
Learn the background
How these networks agree on a map of the internet: BGP in plain English and How routing decisions work. The RPKI article on the blog makes much more sense afterwards.
Frequently asked questions
What is an autonomous system?
A network under one administrative control with its own routing policy: an ISP, a cloud provider, a university, a large company. Each gets an AS number and announces its IP prefixes to neighbouring systems with BGP. The internet is the sum of those announcements.
Who assigns AS numbers?
IANA delegates blocks to the five regional registries (ARIN, RIPE NCC, APNIC, LACNIC, AFRINIC), which assign them to organisations that need to run BGP with more than one provider. Numbers above 65535 are 4-byte ASNs, standard since 2007.
What are private AS numbers?
The ranges 64512–65534 and 4200000000–4294967294 are reserved for internal use (RFC 6996): labs, internal BGP between your own routers, or ISP customer arrangements. They must be stripped before routes reach the public internet, like private IP space.
What’s the difference between an ASN and an IP address?
An IP address identifies a single device or service; an ASN identifies the whole network that a block of IP addresses belongs to. One ASN usually covers thousands or millions of IPs. Put simply: the IP is the house, the ASN is the town it sits in.
Do I need my own ASN?
Almost certainly not. You only need one if you run a network that connects to two or more providers and wants to control its own routing, typically an ISP, a hosting company, or a large business. Everyone else simply uses their provider’s ASN without ever thinking about it.
How many ASNs exist?
Somewhere north of 75,000 networks are active on the internet today. The original format allowed about 65,000 numbers; when those started running low, a larger 4-byte format was introduced in 2007, giving over four billion possible ASNs: far more than we’ll ever need.
Is looking up an ASN safe and legal?
Yes. ASN and registry records are public information, published by the regional internet registries so that networks can contact each other and report problems. Looking one up is completely normal. It’s the same data a network engineer checks every day.