What is an autonomous system? What are ASNs?

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In today’s interconnected world, the smooth functioning of the internet relies on countless networks communicating seamlessly. At the heart of this global connectivity lie Autonomous Systems (AS) and their unique identifiers, the Autonomous System Numbers (ASNs). Whether you’re a network engineer, a developer, or just curious about how internet routing works, understanding ASNs is key to grasping the backbone of modern data transmission.

What Are Autonomous Systems and ASNs?

An Autonomous System is essentially a large network or a group of networks under a single administrative control that presents a unified routing policy to the internet. Think of an AS as a city within a vast country—the city manages its own postal system and traffic routes, ensuring mail and packages (data packets) are efficiently delivered within its borders and to other cities.

An Autonomous System Number (ASN) is a unique identifier assigned to each AS. Much like a city’s postal code, an ASN tells the internet “who’s who” among networks. These numbers come in two primary formats:

16-bit ASNs: Historically, these provided up to 65,536 unique identifiers.
32-bit ASNs: Introduced later to accommodate growing demand, offering billions of possibilities.

ASNs are assigned by the Internet Assigned Numbers Authority (IANA) through regional organizations known as Regional Internet Registries (RIRs).

The Role of ASNs in Internet Routing

The Border Gateway Protocol (BGP) is the core routing protocol that uses ASNs to determine the best paths for data to travel across the internet. Each AS announces the IP prefixes (the “addresses”) it controls to neighboring ASes, and BGP uses this information to build a global map of network paths.

For example, when data needs to travel from one part of the world to another, BGP looks at the ASNs along the route to decide which path will be the fastest and most reliable. This dynamic routing is crucial for handling the ever-changing landscape of internet connectivity, where outages or congestion in one network can prompt a quick reroute via alternative paths.

Why ASNs Matter for Developers and Network Operators

For developers, ASNs aren’t just abstract numbers—they’re a practical tool in network diagnostics and optimization. Here are a few ways ASNs make a difference:

Multihoming: Organizations that connect to multiple internet service providers (ISPs) can use ASNs to manage failover and load balancing.
Peering Arrangements: ASNs help in establishing efficient routes between networks, ensuring data reaches its destination through the most optimal path.
Network Analysis: By examining ASN data, developers can troubleshoot connectivity issues and gain insights into the structure of the internet.

Leveraging ASN Data with IPLocate

At IPLocate, we understand the importance of detailed network data. Our API not only provides geolocation information but also offers comprehensive ASN data—including the ASN itself, the associated IP range, network name, and more.

For instance, using the IPLocate API, you can easily look up ASN details for a given IP address. Here’s a quick example using curl:

curl https://iplocate.io/api/lookup/8.8.8.8?apikey=your_api_key

This request returns structured data about the IP address, including the ASN block. For example:

{
  // ...
  "asn": {
    "asn": "AS7545",
    "route": "123.243.246.0/24",
    "netname": "TPG-INTERNET-AP",
    "name": "TPG Telecom Limited",
    "country_code": "AU",
    "domain": "tpgtelecom.com.au",
    "type": "isp",
    "rir": "APNIC"
  },
  // ...
}

To see all the details our API offers, check out our ASN data section in the documentation.

What is an autonomous system? What are ASNs?

What Are Autonomous Systems and ASNs?

The Role of ASNs in Internet Routing

Why ASNs Matter for Developers and Network Operators

Leveraging ASN Data with IPLocate

Further Reading and External Resources