What is an Autonomous System (AS)?

An Autonomous System (AS) is a large network or group of networks under a single administrative control that presents a unified routing policy to the internet. Every major ISP, cloud provider, enterprise, and content delivery network operates as an autonomous system. Together, they form the building blocks of the global internet.

What Makes a Network "Autonomous"?

The word "autonomous" means the network operates independently. Each AS makes its own decisions about how to route traffic internally, which external networks to connect to, and which routes to accept or reject. The internal details of an AS — its routers, links, and IGP protocols — are invisible to the rest of the internet. Other networks only see the AS as a single entity in BGP routing tables.

Think of autonomous systems like countries in a postal system. Each country manages its own internal mail delivery, but international mail is exchanged through agreed-upon border crossings. In internet terms, BGP is the protocol that handles these border exchanges.

AS Numbers (ASNs)

Every autonomous system is identified by a unique Autonomous System Number (ASN). ASNs were originally 16-bit numbers (0–65,535), but as the internet grew, 32-bit ASNs (up to 4,294,967,295) were introduced in 2007 via RFC 4893.

Some well-known ASNs:

• AS15169 — Google
• AS13335 — Cloudflare
• AS32934 — Facebook (Meta)
• AS16509 — Amazon (AWS)
• AS8075 — Microsoft
• AS714 — Apple

You can look up any ASN to see its announced routes, neighbors, and routing details.

How ASNs Are Assigned

ASNs are allocated by Regional Internet Registries (RIRs), the five organizations responsible for distributing internet number resources in different parts of the world:

• ARIN — North America
• RIPE NCC — Europe, Middle East, Central Asia
• APNIC — Asia-Pacific
• AFRINIC — Africa
• LACNIC — Latin America and Caribbean

To receive an ASN, an organization must demonstrate that it needs to implement a unique routing policy — typically because it connects to multiple upstream providers (multihoming) or provides transit to other networks.

Types of Autonomous Systems

Autonomous systems can be broadly categorized by their role in the routing ecosystem:

• Stub AS — Connects to only one other AS. Most enterprise networks are stub ASes. They have a single upstream provider and do not carry transit traffic.
• Multihomed AS — Connects to two or more ASes for redundancy, but does not pass traffic between them. A company with two ISP connections is multihomed.
• Transit AS — Carries traffic between other autonomous systems. Large ISPs and backbone providers are transit ASes. They form the core infrastructure of the internet.

What an AS Announces

Each AS announces one or more IP prefixes — blocks of IP addresses that it is responsible for. For example, Google (AS15169) announces hundreds of prefixes covering all of Google's IP address space.

When you look up an IP address, the BGP routing table shows you which AS originated the prefix containing that IP, and the AS path that traffic would take to reach it.

Explore Autonomous Systems

Use the looking glass to explore any autonomous system — see its announced prefixes, upstream and downstream neighbors, and routing details:

• Look up AS13335 — Cloudflare
• Look up AS32934 — Facebook (Meta)
• Look up AS2906 — Netflix