Glossary term
Autonomous System Number (ASN)
An Autonomous System Number (ASN) is the globally unique identifier for a network that sets its own routing policy on the internet.
What Autonomous System Number (ASN) means
An Autonomous System Number, or ASN, is the globally unique identifier assigned to an Autonomous System: a collection of IP prefixes under the control of a single administrative entity that presents a common, clearly defined routing policy to the rest of the internet. When people say a network "has its own ASN," they mean that network announces its address space to the global routing table under its own number rather than borrowing an upstream provider's identity.
The internet is not one network; it is tens of thousands of independently operated networks that agree to exchange traffic. Each of those networks is an Autonomous System, and the Border Gateway Protocol (BGP) is the language they use to tell each other which blocks of IP addresses they can deliver traffic to. Every BGP announcement carries an AS path — the ordered list of Autonomous Systems a route has traversed — and that path is built out of ASNs. Without a unique number per network, routers would have no way to detect loops, apply policy, or attribute a route to its origin.
ASNs come in two sizes. The original 16-bit format allowed numbers from 0 to 65535, and that space was effectively exhausted, so the registries moved to 32-bit ASNs, which extend the range into the billions. A network operator obtains an ASN from a Regional Internet Registry — ARIN in North America, RIPE NCC in Europe and the Middle East, APNIC in the Asia-Pacific, LACNIC in Latin America, or AFRINIC in Africa — typically by demonstrating that it will multihome, meaning connect to more than one upstream provider, or otherwise needs an independent routing identity.
In day-to-day operations the ASN is how the internet attributes routing behavior. When a prefix is announced from an unexpected ASN, that is the signal behind a Multi-Origin AS (MOAS) event, one of the classic indicators of a routing leak or hijack. When an analyst wants to know who is responsible for a given IP address, the chain runs from the address to the prefix that contains it to the ASN that originates that prefix to the organization that holds the ASN. Large content and cloud providers operate well-known ASNs that recur constantly in this kind of analysis.
A single organization can hold many ASNs. A global content provider might use distinct numbers for different regions, business units, or acquisitions, while a small enterprise might multihome behind exactly one. The number itself is just an integer; what gives it meaning is the set of prefixes it originates, the peers and upstreams it connects to, and the policies it expresses in BGP. Whisper Canon publishes a page for every ASN observed in the routing table, recording the prefixes it originates, the peering relationships visible in the graph, and the registry metadata attached to it.
ASNs are also the natural unit for reasoning about the shape and resilience of the internet. Counting the ASNs a network peers with gives a rough measure of how well-connected it is; a richly peered ASN is more likely to be operating anycast services, while a network that reaches the world through a single upstream is more fragile. Tracking which ASNs originate which prefixes over time is how routing-security researchers spot hijacks, misconfigurations, and the slow drift of address space between operators. The ASN is, in short, the primary key of the internet's routing layer — the handle every other routing fact hangs off of.
When you read an ASN written as "AS15169," the "AS" prefix is conventional shorthand for "Autonomous System" and the number that follows is the registry-assigned identifier. Canon's URLs use the bare number, so AS15169 lives at /asn/15169. From any ASN page you can pivot to the prefixes it announces, the country its registration is associated with, and the routing conflicts it participates in, making the ASN the central hub of infrastructure analysis on the directory.
Examples in Whisper Canon
Concrete pages in the directory that illustrate Autonomous System Number (ASN).