What Is an ASN (Autonomous System Number)?

Q: What is an ASN (autonomous system number)?

An ASN is a unique identifier for an autonomous system, which is a network or group of networks run under a single routing policy, usually by an ISP, large company, cloud provider, or university. Where an IP address identifies one host, an ASN identifies the whole network that a block of addresses belongs to. BGP uses ASNs to exchange routes between networks.

Q: How do I find the ASN for an IP address?

The quickest way is a WHOIS or RDAP lookup, which returns the ASN, the network owner, and the allocated range for an IP. An IP lookup tool surfaces the same ASN and organization alongside the geolocation. Knowing the ASN tells you which network an address really belongs to.

Q: What is the difference between an ASN and an IP address?

An IP address is the address of a single endpoint, while an ASN labels the network operator that announces a range of IP addresses to the rest of the internet. One ASN can be responsible for many IP blocks. BGP routing reasons about paths between ASNs rather than individual IPs, so the ASN is the network-operator layer above the IP.

Q: Who assigns ASNs?

ASNs are assigned by the five Regional Internet Registries: ARIN for North America, RIPE NCC for Europe and the Middle East, APNIC for the Asia-Pacific, LACNIC for Latin America, and AFRINIC for Africa. An organization that wants its own independent routing policy applies to its regional registry for an ASN.

Q: What is a 32-bit ASN?

ASNs were originally 16-bit numbers with a range of 0 to 65535. As that pool ran low, 32-bit ASNs were standardized in RFC 4893 and later RFC 6793, expanding the range up to 4,294,967,295. The two formats coexist seamlessly, since a 16-bit ASN is just a 32-bit ASN with a small value.

~/sheets/what-is-an-asn.md

The ID Number for a Network
An ASN, or Autonomous System Number, is a unique identifier for an autonomous system — a network (or group of networks) operated under a single, consistent routing policy, typically by an ISP, a large company, a cloud provider, or a university. Where an IP address identifies a single host, an ASN identifies the whole network that a block of IP addresses belongs to. ASNs are the unit that the internet's core routing protocol, BGP (Border Gateway Protocol), uses to exchange routes between these networks.
You can see the ASN behind any address: run our WHOIS Lookup or IP Lookup on an IP and it returns the ASN and the organization that operates that network.
ASN vs IP Address
They sit at different levels. An IP address is the address of one endpoint. An ASN labels the network operator that announces a range of IP addresses to the rest of the internet. One ASN can be responsible for many IP blocks. When BGP decides how traffic gets from one part of the internet to another, it reasons about paths between ASNs, not individual IPs — so the ASN is essentially the "who runs this network" layer above the IP.
Who Assigns ASNs?
ASNs are handed out by the five Regional Internet Registries (RIRs), the same bodies that allocate IP address blocks: ARIN (North America), RIPE NCC (Europe and the Middle East), APNIC (Asia-Pacific), LACNIC (Latin America), and AFRINIC (Africa). An organization that wants to run its own independent routing policy and connect to multiple providers applies to its regional RIR for an ASN.
16-bit vs 32-bit ASNs
ASNs were originally 16-bit numbers, giving the range 0 to 65535. As the internet grew, that pool ran low, so 32-bit ASNs were standardized (RFC 4893, and later RFC 6793), expanding the range up to 4,294,967,295. The two coexist seamlessly today — a 16-bit ASN is simply a 32-bit ASN with a small value. There are also reserved private ASN ranges used inside organizations that are never announced on the public internet.
How to Find the ASN for an IP
The quickest way is a WHOIS or RDAP lookup, which is exactly what our WHOIS tool does — give it an IP and it returns the ASN, the network owner, and the allocated range. Our IP Lookup surfaces the same ASN and organization alongside the geolocation. Knowing the ASN tells you which network an address really belongs to, which is useful for spotting hosting and datacenter ranges, identifying a VPN or proxy provider, and understanding who actually operates an address you are investigating.
Frequently Asked Questions
What is an ASN (autonomous system number)?
An ASN is a unique identifier for an autonomous system, which is a network or group of networks run under a single routing policy, usually by an ISP, large company, cloud provider, or university. Where an IP address identifies one host, an ASN identifies the whole network that a block of addresses belongs to. BGP uses ASNs to exchange routes between networks.
How do I find the ASN for an IP address?
The quickest way is a WHOIS or RDAP lookup, which returns the ASN, the network owner, and the allocated range for an IP. An IP lookup tool surfaces the same ASN and organization alongside the geolocation. Knowing the ASN tells you which network an address really belongs to.
What is the difference between an ASN and an IP address?
An IP address is the address of a single endpoint, while an ASN labels the network operator that announces a range of IP addresses to the rest of the internet. One ASN can be responsible for many IP blocks. BGP routing reasons about paths between ASNs rather than individual IPs, so the ASN is the network-operator layer above the IP.
Who assigns ASNs?
ASNs are assigned by the five Regional Internet Registries: ARIN for North America, RIPE NCC for Europe and the Middle East, APNIC for the Asia-Pacific, LACNIC for Latin America, and AFRINIC for Africa. An organization that wants its own independent routing policy applies to its regional registry for an ASN.
What is a 32-bit ASN?
ASNs were originally 16-bit numbers with a range of 0 to 65535. As that pool ran low, 32-bit ASNs were standardized in RFC 4893 and later RFC 6793, expanding the range up to 4,294,967,295. The two formats coexist seamlessly, since a 16-bit ASN is just a 32-bit ASN with a small value.
Last updated: April 2026