What is an ASN, and what does it identify?¶
title: "What is an ASN, and what does it identify?" status: "complete" source_document: "https://docs.google.com/document/d/1WiJr4v0-WbDpEQ_yOwszyKhjUnNzos0cAMGlAadTmys/edit?usp=drivesdk"
What is an ASN, and what does it identify?¶
An ASN is an Autonomous System Number.
It identifies an autonomous system: a network, or group of connected networks, that presents a common routing policy to the wider internet. Internet providers, cloud companies, universities, large organisations and content-delivery networks may operate their own autonomous systems.
Networks use routing announcements to tell other networks which IP address ranges can be reached through them. Those announcements are associated with ASNs. An IP lookup may therefore show an ASN for the network announcing the route containing the address.
The ASN can help identify the organisation currently carrying or announcing traffic for an address range. That may point towards an internet provider, hosting company, mobile operator or another network involved in delivering the service.
But the ASN doesn't normally identify the subscriber using one individual IP address. A provider’s autonomous system may carry traffic for millions of customers. It may also announce address space registered to another organisation or provide connectivity for businesses that hold their own address ranges.
The organisation registered for an IP range and the organisation associated with the announcing ASN aren't necessarily the same. Resellers, managed services, subsidiaries and upstream providers can create additional layers between the registered address holder, the announcing network and the customer using the address.
ASN information is also time-sensitive. Routing arrangements can change, and a different autonomous system may announce a range at another time. A current ASN result may not describe the route or provider relationship that existed during an earlier event.
An ASN identifies a network involved in internet routing. It can help classify the infrastructure and identify a possible data holder, but it doesn't establish the customer, device, physical location or person responsible for activity involving an individual IP address.