Who is my ISP
What Is an ASN?
An ASN, or autonomous system number, is how the internet identifies networks, in the same way an IP address identifies a connection. Your ISP has an ASN, lookup tools show it next to the provider name, and once you know what it means, those four or five digits stop looking like noise.
What is an autonomous system?
An autonomous system (AS) is a network, or a group of networks, run by one organization with a single, unified routing policy. The largest ones are the backbone networks of internet providers; smaller ISPs, universities, and big companies run their own too. Autonomous systems exchange routing information with each other using the Border Gateway Protocol (BGP), and that continuous conversation is what makes the global internet work.
What does an ASN look like?
An ASN is written as the letters "AS" followed by a number, like AS7922 for Comcast or AS15169 for Google. Numbers were originally 16-bit, which capped them at 65,535, and the last 16-bit blocks were handed out in 2011. A 32-bit extension (RFC 6793) solved that, which is why modern ASNs can be much longer.
ASNs are assigned by the five regional internet registries: ARIN in North America, RIPE NCC in Europe and the Middle East, APNIC in Asia-Pacific, LACNIC in Latin America, and AFRINIC in Africa. Roughly 79,000 are visible in the global routing table today.
Why does your ISP have an ASN?
Every public IP address sits inside a block that is announced to the rest of the internet by exactly one autonomous system. The ASN is that network's identity, and the registries record which organization owns it. That chain is what makes ISP lookups possible: your IP maps to an ASN, and the ASN maps to an organization name, such as "Comcast Cable Communications, LLC." You can see this chain in action on our homepage, and we explain the name-vs-bill mismatch in what is my ISP number.
How to find your ASN
- Our Who Is My ISP tool shows your provider together with its ASN.
- On macOS or Linux, run whois on your public IP; the "origin" field names the ASN.
- The regional registry databases (for example ARIN's search) let you look up any ASN and see its announced address blocks.
Does my ASN change?
Not on a normal home connection: your ISP announces the same network regardless of which individual IP you get. It does change when your traffic takes a different path to the internet, for example when you connect to a VPN or a mobile network.
Residential ISPs, hosting companies, and VPNs
The same mechanism identifies every network on the internet, not just consumer providers. Cloud providers and data centers have ASNs, and so do VPN companies. That is why an ISP lookup run while a VPN is connected shows the VPN provider's ASN: your traffic genuinely exits through their network. It is also why websites can tell a residential connection apart from a data center one, a distinction used in fraud checks and some content licensing.
Why should you care about an ASN?
For everyday browsing, you do not need to. But it helps explain a few things you will notice: why an ISP lookup sometimes names a company you have never heard of, why a website can tell you are on a VPN or a hosting provider, and why support engineers ask which network your connection is on. The ASN is the common language for all of those conversations.