What is an ASN?
The internet is not one network. It is roughly eighty thousand independent networks that have agreed to exchange traffic. Each of them is an autonomous system, and each carries a number — its ASN — that identifies it in the global routing table.
What an autonomous system is
An autonomous system is a collection of IP address blocks under one administration with a single routing policy. Your ISP is one. So is every mobile operator, every large hosting company, most universities, many banks, and Cloudflare, Google and Amazon several times over.
Each announces to its neighbours, through the Border Gateway Protocol, which address blocks it can reach. Those announcements, propagated worldwide, are what makes a packet from one continent find a server on another.
Why the number is useful
The ASN is the most reliable way to say which operator an address belongs to.
- Geolocation is an estimate; ASN ownership is a registry record.
- Abuse reports go to the autonomous system responsible for the block.
- Network-type detection — home, hosting, VPN — is largely a question of which ASN an address sits in.
- Content delivery and traffic engineering decisions are made per autonomous system.
When the provider name on a lookup looks unfamiliar, the AS number is the thing to search for. It leads to the registry record and the actual owner.
Reading your own
The AS number on the home page belongs to whichever network your address sits in. If you are on a home connection it is your ISP. If you are on a VPN it is the data centre operator the VPN rents from. If it is Amazon, Google or Microsoft, you are connecting from a cloud instance or a service that proxies through one.
The numbers themselves
Originally 16-bit, so 0 to 65535. That ran short, and 32-bit numbers were introduced in 2007, extending the range past four billion. Both are in use; a number above 65535 simply means it was allocated more recently. Some ranges are reserved for private use inside organisations and never appear on the public internet.
Frequently asked questions
What does AS3320 mean?
It is the identifier of one autonomous system — in that case Deutsche Telekom's network. The number is unique worldwide and assigned by a regional internet registry.
How many autonomous systems exist?
Somewhat over 80,000 are visible in the global routing table at any moment, out of a much larger pool of allocated numbers.
Can I get my own ASN?
Yes, if you run a network that connects to at least two others and holds its own address space. Registries charge a membership fee. It is routine for ISPs and large companies, and impractical for a household.
Why does my ASN not match my provider's name?
Resellers use their upstream's addresses, and groups register blocks to a parent company. The AS number names the network that announces the route, which is not always the brand on your bill.