What is an ASN?

// Introduction

An ASN (Autonomous System Number) is a unique identifier assigned to a group of IP networks managed under a single routing policy. ASNs are the backbone of how traffic is directed across the global internet.

When you look up an IP address and see "AS15169 — Google LLC," you are seeing the Autonomous System that owns and routes that IP address block. Understanding ASNs helps you identify who is behind any IP address — whether it belongs to an ISP, a cloud provider, a VPN service, or a university.

// What is an Autonomous System?

The internet is not a single network — it is a collection of thousands of independently managed networks called Autonomous Systems (AS). Each AS is a network (or group of networks) operated by a single organisation under a unified routing policy.

Examples of Autonomous Systems include:

• Internet Service Providers (ISPs) like Deutsche Telekom, Comcast, or Orange
• Cloud providers: Amazon Web Services, Google Cloud, Microsoft Azure
• Content delivery networks: Cloudflare, Akamai, Fastly
• Universities and research institutions
• Large corporations running their own global networks
• Government agencies and military networks

// BGP — The Routing Protocol

Autonomous Systems communicate with each other using BGP (Border Gateway Protocol), sometimes called the "protocol that holds the internet together." BGP is a path-vector protocol that determines how traffic flows between ASes.

Each AS announces to its BGP peers which IP address ranges (called prefixes) it is responsible for. These announcements propagate across the entire internet, allowing every router to build a routing table that maps IP prefixes to the AS paths to reach them.

BGP is designed for policy-based routing — an AS can choose not to route traffic for competitors, prefer certain upstream providers, or apply security filters. This flexibility makes BGP powerful but also a potential source of routing incidents when misconfigured.

// ASN Format and Assignment

ASNs were originally 16-bit numbers (1–65535), providing about 65,000 unique identifiers. As demand grew, 32-bit ASNs (RFC 6793) were introduced in 2007, expanding the space to over 4 billion unique numbers.

Private ASNs (64512–65534 for 16-bit, 4200000000–4294967294 for 32-bit) can be used for internal routing without being announced to the global internet — similar to private IP addresses.

// Who Assigns ASNs?

The Internet Assigned Numbers Authority (IANA) delegates ASN allocation to five Regional Internet Registries (RIRs), each responsible for a geographic region:

To obtain an ASN, an organisation must apply to their regional RIR, demonstrate a need for multi-homed routing, and pay an annual membership fee.

// Well-Known ASNs

// What an ASN Reveals About an IP

Looking up an IP's ASN can tell you:

• Who owns the IP range — ISP, hosting company, VPN provider, or enterprise
• Residential vs data-center IP — streaming services use this to block VPNs
• Geographic routing — which region's infrastructure is handling the traffic
• Abuse attribution — security teams use ASN data to block known bad actors
• Network peering — whether the AS connects to others via IXPs or private peering

IP addresses within the same ASN share the same operator and routing policy, but may be used by many different end customers (especially for ISP and hosting ASNs).

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