What Is an ASN? Autonomous System Numbers Explained

ASN stands for Autonomous System Number. It is a number used to identify an autonomous system on the internet — broadly, a network or group of IP prefixes operated under a common routing policy. ASNs are used with BGP to help networks exchange routing information and determine how internet traffic can reach different destinations.

This article explains what that means in plain language, how an ASN differs from an IP address or an ISP, and what an ASN does and does not tell you.

What Does ASN Mean?

ASN is short for Autonomous System Number. An autonomous system is a collection of IP prefixes (blocks of IP addresses) presented to the rest of the internet under a common routing policy. The ASN is the identifier for that autonomous system in inter-domain routing, which is the process networks use to exchange reachability information with each other.

ASNs are usually written with the letters "AS" followed by a number. A made-up example in a range reserved for documentation is AS64496. It is not a real network.

An ASN is not an IP address, it is not always an ISP, and it does not identify an individual person. If you are new to addressing, you can first read about how IP addresses work.

Why Do ASNs Exist?

The internet is not one network. It is made of many independently operated networks that connect to each other. For traffic to cross from one to another, networks need a way to tell each other which IP prefixes they can reach. ASNs provide the identifiers used in that routing system, so networks can say which autonomous system announced a prefix and which path leads toward it.

One analogy is a postal system in which each regional operator has an ID and shares which addresses it can deliver to. This is only an analogy. An ASN is not a physical router, a server, or a place. It is a label for an administrative and routing unit.

What Is an Autonomous System?

At a high level, an autonomous system is:

• A network, or a group of IP prefixes
• Managed under a common routing policy
• A participant in internet routing
• Able to announce routes to other networks

Organizations that operate autonomous systems include internet service providers, cloud providers, hosting companies, universities, large companies, content delivery and network providers, and government or research networks. Not every organization needs its own ASN. Many businesses and households simply use address space provided by an upstream network.

What Is BGP?

BGP stands for Border Gateway Protocol. It is the protocol networks use to exchange routing information between autonomous systems. At a beginner level, networks advertise which IP prefixes they can reach, and other networks learn about those routes and use them when deciding where to send traffic.

Routing decisions can depend on policy and on path information, such as business relationships between networks and the routes available. BGP is not simply a tool that picks the geographically shortest path. This article stays at the overview level and is not a configuration guide.

ASN vs IP Address

An IP address identifies an address, such as the one a device or network interface uses for communication. An ASN identifies an autonomous system in inter-domain routing. They are different kinds of identifiers.

Many IP prefixes can be associated with a single ASN, and each prefix contains many individual addresses. An IP lookup may map a public IP address to the ASN currently associated with the network that announces or routes that prefix. That mapping can change over time. To understand which addresses are visible from the internet, see public and private IP addresses.

ASN vs ISP

ASN and ISP are not synonyms. Consider these points:

• Many ISPs operate one or more ASNs.
• Organizations that are not consumer ISPs can also operate ASNs.
• A large organization may operate multiple ASNs.
• Network relationships and routing can be more complicated than a simple "one ISP equals one ASN" mapping.

Because of this, the ASN shown in a lookup describes a network, while the ISP is a provider. They are often related but are not the same thing. For more detail, see how ISP identification works.

Can One Company Have Multiple ASNs?

Yes. A company may operate more than one ASN for reasons such as:

• Separate networks
• Acquisitions
• Different regions
• Different routing policies
• Different services or infrastructure

These are examples, not a complete list. Since a company can hold several ASNs, an ASN should not automatically be treated as a unique identifier for an entire corporate entity.

Can Multiple IP Addresses Use the Same ASN?

Yes. An autonomous system can announce many IP prefixes, and each prefix can contain many individual addresses. As a result, many users, servers, or services may appear under the same ASN.

An ASN does not uniquely identify a user, a device, a household, or a website.

How Can I Find the ASN for My IP?

IP and network lookup databases can associate a public IP address with routing and network information, including the ASN. The WhatIsMyIP homepage may display your public IP address, ISP, organization or network, ASN, and approximate location, depending on the data available. The ASN field may not appear for every lookup. You can read more about the site on the About page.

Check your IP address and network details

Why Might Different ASN Lookup Tools Show Different Results?

Two tools can show different results for the same address without either being broken. Common reasons include:

• Database update timing
• Routing changes
• Different data sources
• IP prefix reassignment
• Different interpretation or labeling
• More-specific route announcements, where a smaller block within a larger one is announced separately
• Network restructuring

A disagreement does not automatically mean that one tool is wrong or fraudulent. It is often a sign that the data is changing or is being labeled differently.

Does an ASN Reveal My Location?

An ASN identifies a network in the routing system, not a precise physical location. A single network may operate in multiple cities, across a country, or across several countries.

ASN and network information can contribute context to IP location estimates, but it should not be treated as GPS or as a street address.

Does a VPN Change the ASN Websites See?

When you connect through a VPN, websites normally receive the VPN server's public IP address instead of your usual public-facing one. A lookup may therefore show the ASN associated with the VPN server or the datacenter network it uses. Your underlying ISP or network still carries your connection to the VPN.

This is not a rule for every setup, and VPN configurations vary. A VPN does not guarantee anonymity. A datacenter ASN does not always mean a VPN, and an ASN alone cannot always reveal one. The WhatIsMyIP homepage has an optional VPN and proxy check, but detection is not guaranteed to be accurate.

Can an ASN Identify a Person?

No. An ASN itself does not identify an individual person. It identifies an autonomous system, meaning a network in internet routing, and many users and services may use addresses routed through the same ASN.

An ordinary ASN lookup does not provide a subscriber's name, home address, or account number. That describes what a normal public lookup shows. It does not mean that providers or authorized authorities could never correlate separate records. For how information may be handled on this site, see the Privacy Policy.

Are ASNs Permanent?

An ASN is an assigned identifier used by a network, but internet routing and organizational arrangements change over time. Networks may change their routing, move prefixes around, acquire infrastructure, reorganize, or use different ASNs for different purposes.

This means an IP address will not necessarily stay associated with the same ASN forever. If you are comparing results over time, expect some variation.

Frequently Asked Questions