ASN Meaning: What an Autonomous System Number Is

You ran an IP lookup, saw a field labeled ASN with a value like AS15169, and wondered what it actually means. An ASN, or Autonomous System Number, is the unique ID for the network that owns and routes that IP address. This guide explains what an ASN is, how ASNs work with BGP, the difference between public and private ASNs, how they are assigned, and how to look one up.

What is an ASN (Autonomous System Number)?

An Autonomous System Number (ASN) is a globally unique number that identifies an autonomous system: a network, or group of IP ranges, run under a single routing policy by one organization. ASNs let independent networks exchange routing information across the internet. Each ASN is written with an AS prefix, such as AS15169 for Google.

Think of the internet as a network of networks. Each one needs a stable, unique name so the others know how to reach it, and the ASN is that name. Individual IP addresses come and go, but the ASN identifies the operator responsible for a whole block of addresses.

What is an autonomous system?

An autonomous system (AS) is a collection of connected IP address ranges, called prefixes, that operate under one clearly defined routing policy and administration. An internet service provider, a cloud platform, or a large enterprise each runs its own autonomous system. The ASN is the identifier that represents that system to every other network on the internet.

Comcast, Amazon Web Services, and Cloudflare each operate one or more autonomous systems. When you load a website, your request usually crosses several of them before it reaches the server, and each handoff is decided using ASNs.

Autonomous system vs ASN: what is the difference?

An autonomous system is the network itself: the set of IP prefixes one organization routes under a single policy. The ASN is the number that identifies that network to the rest of the internet. OVH's network is an autonomous system; AS16276 is its ASN. People use the terms interchangeably in practice, but the AS is the thing and the ASN is its name.

The distinction matters when you read routing data. A route announcement comes from an autonomous system, and the AS_PATH that travels with it is a list of ASNs. If you can name the number, you can name the network behind it.

What does ASN stand for?

In networking, ASN stands for Autonomous System Number, the globally unique identifier for a network that routes traffic on the internet. That is the meaning this guide covers. The same three letters appear in other fields, which causes confusion when you search for the term.

• Networking: Autonomous System Number, the subject of this guide.
• Nursing: Associate of Science in Nursing, a two-year nursing degree.
• Logistics: Advanced Shipping Notice, a delivery notification document.
• Biochemistry: Asn is the three-letter code for the amino acid asparagine.

If you landed here from an IP lookup, a firewall log, or a routing table, the networking meaning is the one you want.

Why do ASNs exist? ASNs and BGP routing

ASNs exist so independent networks can route traffic between each other. The Border Gateway Protocol (BGP) uses ASNs to announce which IP ranges each network can reach. Routers collect those announcements into a global routing table, then use it to pick the best path from one autonomous system to the next.

Every AS effectively tells its neighbors, "these prefixes are reachable through me." Those announcements propagate across the internet until every BGP router knows which ASN to hand a packet to next. Without ASNs, there would be no consistent way to identify who announces a route, and global routing would not scale.

2-byte vs 4-byte ASNs (ASN format)

ASNs come in two sizes. The original 2-byte (16-bit) format runs from 0 to 65,535, about 65,000 numbers. To meet demand, the 4-byte (32-bit) format was added, running from 0 to 4,294,967,295, over 4 billion numbers. Both are written the same way: AS followed by the number.

The move to 4-byte ASNs (defined in RFC 6793) happened because the 2-byte pool was running low. For backward compatibility, older routers show any 4-byte ASN they cannot interpret as AS23456, a placeholder called AS_TRANS. You may also see 4-byte ASNs written in asdot notation, such as 1.10, instead of a single asplain number.

Public vs private ASNs

Public ASNs are globally unique and used to route traffic across the public internet. Private ASNs are reserved for internal use and never appear in the global routing table. RFC 6996 reserves 64512 to 65534 for 16-bit private ASNs and 4200000000 to 4294967294 for 32-bit private ASNs.

Private ASNs suit large organizations that run BGP internally but do not need a globally unique identity for every segment. A few numbers are reserved outright: AS0 is reserved, and AS65535 and AS4294967295 are set aside and should not be used in routing. For the full breakdown of ranges and formats, see public vs private ASNs.

• 0 to 65,535: the original 2-byte (16-bit) ASN range.
• 64,512 to 65,534: private use within the 2-byte range (RFC 6996).
• 0 to 4,294,967,295: the expanded 4-byte (32-bit) ASN range.
• 4,200,000,000 to 4,294,967,294: private use within the 4-byte range (RFC 6996).

How are ASNs assigned?

ASNs are handed out through a tiered system. IANA allocates blocks of ASNs to the five Regional Internet Registries, ARIN, RIPE NCC, APNIC, LACNIC, and AFRINIC, which then assign individual ASNs to network operators that meet their criteria. Most organizations request an ASN through their regional registry or their upstream provider.

You generally need your own ASN when you run a multi-homed network, meaning you connect to more than one provider and want to control your own routing. A single-homed network usually does not need one. For the ASN ranges and the full assignment process, see how ASNs are assigned.

How many ASNs are there?

About 120,000 ASNs are currently assigned to network operators worldwide, out of more than 400,000 that IANA has delegated to the regional registries. That is still a tiny share of the 4-byte space, which holds over 4 billion possible ASNs, so the supply is nowhere near running out.