What is an ASN, and what does an ISP lookup show?
An ASN (Autonomous System Number) is a number that identifies one network on the internet, such as a broadband provider, a mobile operator or a large company. An IP lookup shows the ASN so you can see who runs the network the address belongs to.
What an autonomous system is
The internet is a collection of thousands of separate networks. Each one that decides its own routing is called an autonomous system and is given a number, such as AS64500. Networks use these numbers to tell each other which addresses they can reach.
What you see in a lookup
A lookup shows several related details:
- ASN: the number of the network that announces the address.
- Organisation or ISP: the company the number is registered to.
- Network range: the block of addresses it belongs to.
Reading the results
A home or mobile address normally shows a consumer provider. A server often shows a hosting or cloud company, which is a strong sign the traffic comes from a website, script or VPN rather than a person at home. That is useful when you are deciding whether a visitor or a login is likely to be genuine.
Where to learn more
The ASN on IPMonkey links to a public routing toolkit that lists the ranges a network announces and the networks it connects to.
Common questions
Is an ASN the same as an ISP?
Not quite. Many ISPs have one or more ASNs, and many companies that are not ISPs have their own ASN too.
Can the ASN tell me who is using an address?
It shows which organisation runs the network, not the person using it. Only that organisation can link an address to a customer.
Why does my address show a hosting company?
You are probably connected through a VPN, proxy or server, whose address belongs to a hosting provider.