What Is an ASN (Autonomous System Number)?
What Is an Autonomous System?
An Autonomous System (AS) is a large network or collection of networks under a single administrative control that presents a common routing policy to the internet. Each AS is identified by a unique number called an ASN (Autonomous System Number).
Think of the internet as a network of networks. Each major ISP, cloud provider, university, and large enterprise operates its own AS. These autonomous systems interconnect with each other to form the global internet, exchanging routing information so that data can travel from any network to any other network.
Examples of well-known autonomous systems:
- AS15169 - Google
- AS13335 - Cloudflare
- AS16509 - Amazon (AWS)
- AS8075 - Microsoft
- AS32934 - Facebook (Meta)
BGP: How Autonomous Systems Communicate
BGP (Border Gateway Protocol) is the routing protocol that autonomous systems use to exchange routing information with each other. It is often called the "routing protocol of the internet" because it determines how data travels between networks.
Here is how BGP works at a high level:
- Each AS announces (advertises) the IP prefixes (address blocks) that it owns to its neighboring ASes.
- Neighboring ASes propagate these announcements to their own neighbors, building a global routing table.
- When a packet needs to travel from AS A to AS B, BGP determines the best path through intermediate ASes based on policies, path length, and other attributes.
BGP relationships between ASes fall into these categories:
- Transit: A customer AS pays a provider AS for internet access. The provider carries the customer's traffic.
- Peering: Two ASes agree to exchange traffic directly (often for free) at an Internet Exchange Point (IXP). This reduces latency and transit costs.
- Upstream/Downstream: An AS that provides connectivity is "upstream"; the AS receiving it is "downstream."
Who Assigns ASNs?
ASNs are allocated by the same hierarchy that manages IP addresses:
| Organization | Role | Region |
|---|---|---|
| IANA | Global ASN pool management | Worldwide |
| ARIN | Regional allocation | North America |
| RIPE NCC | Regional allocation | Europe, Middle East, Central Asia |
| APNIC | Regional allocation | Asia-Pacific |
| LACNIC | Regional allocation | Latin America, Caribbean |
| AFRINIC | Regional allocation | Africa |
To obtain an ASN, an organization must demonstrate that it has a legitimate need for a unique routing policy and that it peers with (or plans to peer with) multiple upstream providers.
16-Bit vs 32-Bit ASNs
Originally, ASNs were 16-bit numbers, providing a range of 0 to 65,535. As the internet grew, this space became insufficient, leading to the introduction of 32-bit ASNs:
| Type | Range | Total Available | Format |
|---|---|---|---|
| 16-bit ASN | 1 – 65,534 | ~65,000 | AS15169 |
| 32-bit ASN | 65,536 – 4,294,967,294 | ~4.2 billion | AS394842 |
Reserved ranges include:
- AS0: Reserved, not used
- AS64496–64511: Reserved for documentation and examples
- AS64512–65534: Reserved for private use (similar to private IP addresses)
- AS23456: "AS_TRANS" - used for backward compatibility during the 16-to-32-bit transition
Looking Up ASN Information
You can look up ASN details using several methods:
- Our ASN Lookup Tool: Use our ASN Lookup to find the organization, country, and IP prefixes associated with any ASN.
- WHOIS: Run
whois AS15169to query the appropriate RIR database. - BGP looking glasses: Tools like RIPE RIS, RouteViews, and Hurricane Electric's BGP Toolkit show real-time BGP routing data.
- IP to ASN: You can find which ASN an IP belongs to:
whois -h whois.cymru.com " -v 8.8.8.8"