What Is an ASN?

An ASN is an Autonomous System Number, a unique number that identifies one network on the internet, such as the network of an internet provider, a university or a large company. When you ask what an ASN is, the short answer is: it is the number that tells other networks "this group of IP addresses belongs to this operator and follows this routing policy". Every IP address you meet online sits inside such a network, and the ASN is the label of that network. This guide explains what an ASN is, how it relates to your IP address and to BGP, how ASNs are assigned and how to find the ASN behind your own connection.

You do not need to know any ASN to use the internet. But the ASN is useful when you read an IP address lookup, check who owns an address or look at why a website treats your connection in a certain way.

What Is an Autonomous System?

An autonomous system (AS) is a connected group of one or more IP address ranges, run by one or more network operators, that has a single and clearly defined routing policy. This is the definition in RFC 1930, the guideline the IETF published in 1996 for the creation, selection and registration of an AS (IETF, 1996).

Routing policy means how a network decides where to send traffic and which neighbours to trust. An AS is the unit that applies one such policy. RFC 1930 also states that an AS has a globally unique number, the ASN, which is used both in the exchange of routing information between neighbouring networks and as an identifier of the AS itself.

How Does an ASN Relate to an IP Address?

An IP address belongs to an IP prefix, which is a block of addresses, and each prefix is announced to the internet by one AS. So the chain is: your device has an IP address, the address sits in a prefix, and the prefix is announced by the ASN of the network that owns or routes it. When a lookup says that your IP address belongs to a certain provider, it has followed this chain from the address to the ASN to the operator behind it.

This is the reason why the owner of an IP address can look different from what you expect. A VPN server, for example, often sits in a data centre network. A lookup then names the data centre operator and not the VPN brand. Our IP address lookup guide shows what a lookup can and cannot tell you.

What Does an ASN Do in Internet Routing?

ASNs let networks exchange routes with each other. The protocol for this is the Border Gateway Protocol (BGP), which the IETF describes as an inter-autonomous-system routing protocol (RFC 4271, 2006). Each network announces which prefixes it can reach, and its neighbours pass the information on. The list of ASNs that a route has crossed is carried in the AS_PATH attribute, so a router can see through which networks the traffic would travel.

In simple words: IP addresses say where a device is, and ASNs say which network is in charge of that part of the internet. BGP is how those networks tell each other the way.

How Long Is an ASN? 16-Bit and 32-Bit ASNs

ASNs were first 16-bit numbers, which allowed at most 65,535 values. RFC 1930 describes this limit and warns that the number space is finite (IETF, 1996). The IETF later defined ASNs with four octets, which means 32 bits, and described how BGP carries them (RFC 6793, 2012). Both forms are in use today.

Some ranges are reserved for private use. RFC 6996 sets aside 64512 to 65534 and 4200000000 to 4294967294 for private ASNs (IETF, 2013). Operators use them inside their own setup, and they do not appear as public networks on the internet.

Who Assigns ASNs?

ASNs are assigned in a chain: the Internet Assigned Numbers Authority (IANA) hands blocks of numbers to five Regional Internet Registries, and these registries assign numbers to network operators. The five registries are AFRINIC, APNIC, ARIN, LACNIC and RIPE NCC (IANA, 2026). Each registry covers a part of the world. The IANA list shows which block belongs to which registry.

Who Needs Their Own ASN?

Most people and most small sites do not need their own ASN. RFC 1930 says a single-homed site, one that connects to just one provider, does not need a separate AS because it shares the routing policy of that provider. The RFC names the multi-homed site, which connects to more than one provider, as almost the only case where an operator should create an AS number of its own (IETF, 1996).

If you want an ASN for your own network, you ask the Regional Internet Registry of your region, and it applies its own rules and fees. This guide cannot list those rules, because they differ by registry and change over time.

How Do I Find My ASN?

You find your ASN by looking up your public IP address and reading the network information of the result. Follow these steps:

  1. Open the SpeedIQ test page and run the IP Geolocation check to see your public IP address.
  2. Run the ASN Lookup check. It shows the name of the organisation that owns your IP address. It does not show the number.
  3. To get the number, enter your IP address into the lookup service (WHOIS or RDAP) of a Regional Internet Registry or any public IP data service. The result lists the ASN together with the network name and the prefix.
  4. Repeat the steps with your VPN switched on. The VPN leak test shows which address and owner websites see.

The ASN you find belongs to the network that carries your traffic, which is usually your provider. A mobile connection, a home connection and a VPN each show a different ASN.

Why Does My ASN Matter for Privacy and Blocking?

Websites and security tools often look at the ASN of a visitor to guess what kind of network it is. An address from a home provider looks different from an address in a data centre. Some services treat data centre networks with more care, because automated tools often run from there. We describe this in our guide to the IP reputation check.

An ASN does not identify you as a person. It names a network, often shared by thousands of customers. It does, however, reveal your provider and a rough area, and together with other values it adds to your browser fingerprint. For a full check of what your connection shows, use our online privacy check guide.

ASN vs IP Address vs ISP: What Is the Difference?

TermWhat it isExample of a question it answers
IP addressThe address of a device or gateway on the internetWhere is the traffic sent?
ASNThe number of the network that announces a block of addressesWhich network is in charge of this address range?
ISPThe company that sells you accessWho do I pay for my connection?

One ISP can run several ASNs, for example after a merger or for different services. A VPN or hosting company has an ASN as well, even though it is not an ISP in the usual sense.

Summary

An ASN is the unique number of an autonomous system, a network with one routing policy. IP addresses belong to prefixes, prefixes are announced by an ASN and BGP carries the routes between ASNs. Regional Internet Registries assign the numbers. To find yours, look up your public IP address and read the network data.

FAQ

What does ASN stand for?

ASN stands for Autonomous System Number. It is the unique number of a network that has its own routing policy on the internet. In education, ASN can also stand for other terms such as a nursing degree, but this guide is about networks.

How do I find an ASN number?

Look up your public IP address in the WHOIS or RDAP service of a Regional Internet Registry or in a public IP data service. The result shows the ASN and the network name. The SpeedIQ ASN Lookup check shows the name of the owner but not the number.

How do I get an ASN?

You apply to the Regional Internet Registry of your region. RFC 1930 explains that an operator should create its own AS number mainly when its network connects to more than one provider. Rules and fees depend on the registry.

Is an ASN the same as an IP address?

No. An IP address identifies a device or gateway. An ASN identifies the network that announces a block of IP addresses. One ASN usually covers many IP prefixes.

Can an ASN identify me?

No, an ASN names a network, not a person. It can show your provider and a rough area, and websites use it to judge what kind of network a visit comes from.

Sources