ASN Lookup Tool

What is an Autonomous System?

The internet isn't one giant network. It's thousands of smaller networks all connected together.

Each of these smaller networks is called an Autonomous System, or AS for short.

Think of an AS as a neighborhood in a city. Just like each neighborhood has its own streets and management, each AS has its own IP addresses and routers managed by one organization.

Your internet service provider runs an AS. That's how you get online. Amazon runs an AS for AWS. Google has one too. Universities have them. Big companies have them.

The word "autonomous" means independent. The company running an AS gets full control over how it operates.

They decide which routes traffic takes inside their network. They choose which other networks to connect with. They set all the rules.

Every AS gets a unique identification number called an ASN. These numbers are handed out by five regional organizations around the world: ARIN, AFRINIC, LACNIC, RIPE, and APNIC.

No two networks can have the same ASN. It's like a social security number, but for networks.

When you look up an ASN, you're essentially looking up the ID card of a network to see who owns it and what IP addresses they control.

IP Address Space in an AS

An autonomous system doesn't just have a number. It also controls actual IP addresses that devices use to communicate.

These IP addresses come in organized blocks called prefixes. A prefix like 203.0.113.0/24 contains 256 individual IP addresses. The organization running that AS decides how to use them. Some addresses run their network infrastructure. Others get distributed to customers. Some stay available for expansion.

Large networks typically manage dozens or even hundreds of different prefixes. They accumulate these blocks over time through new allocations, company acquisitions, or geographic expansion. Our tool displays all the prefixes an AS currently announces, giving you a complete picture of their IP address holdings and network size.

What is an ASN (Autonomous System Number)?

Every network on the internet needs its own unique identification number. This number is called an ASN.

Think of it like a passport number or vehicle registration. It identifies which organization controls which network. Regional internet registries assign these numbers to ensure no duplicates exist worldwide. The five registries are ARIN (North America), RIPE (Europe/Middle East), APNIC (Asia Pacific), LACNIC (Latin America), and AFRINIC (Africa)

ASN Format

ASNs come in two formats. The original 16-bit format ranges from 1 to 65,535 where 1023 of them were reserved for private use. As the internet grew, just like how IPv6 was introduced, 32-bit ASNs were introduced, extending the range to over 4 billion. Both are in use today. They are usually written with "AS" in front, like AS15169 for Google or AS16509 for Amazon.

What is BGP (Border Gateway Protocol)?

BGP is the routing protocol that holds the entire internet together. Think of it as the postal system of the internet. Just like post offices need to know how to route mail between cities, BGP helps routers know how to send data between different networks.

When you send data across the internet, it doesn't go directly to its destination. It hops through multiple networks (multiple ASes) to get there. BGP is what makes those hops possible. It's how networks talk to each other and share routing information.

Without BGP, networks would be isolated islands with no way to communicate. BGP is what connects these islands into the global internet we use every day.

How ASN and BGP Work Together

Each AS uses BGP to announce which IP address ranges it controls. This is called "advertising prefixes". For example, Google's AS15169 announces to its neighbors: "Hey, I can reach these IP addresses. Send me any traffic destined for them."

Those neighbor networks store that information and pass it along to their neighbors. This continues across the internet, creating a massive map of which ASN can reach which IPs.

When you try to access a website, your ISP's routers look at the destination IP and check their BGP routing table. They see which path of ASNs will get your data there.

Your data packet gets tagged with the source ASN (your ISP) and travels through a chain of ASes until it reaches the destination ASN. This chain is called the AS path.

BGP routing decisions aren't just about the shortest path. They consider business relationships between networks, network performance, traffic policies, and the health of connections.

This is why two people in the same city might take completely different AS paths to reach the same website. Their ISPs have different peering agreements and routing policies.

Understanding how ASN and BGP work together is essential for network troubleshooting, security analysis, and understanding internet infrastructure.

How BGP Uses ASNs for Internet Routing

When you send data across the internet, it doesn't go directly to its destination. It hops through multiple networks (multiple ASes) to get there. BGP (Border Gateway Protocol) is the system that makes this work. Think of it as the GPS of the internet, finding paths between autonomous systems.

Each AS uses BGP to announce which IP addresses it can reach. Those neighbors pass that information along, and so on. When your data needs to reach a specific IP, routers use this BGP information to figure out which path of ASes will get it there. The route your data takes is called the AS path.

BGP routing decisions consider business relationships, network performance, and policies set by each AS, not just the shortest path. This is why understanding ASN relationships matters for anyone working with network infrastructure.

Types of Autonomous Systems

Not all autonomous systems are created equal. There are different types of AS based on how they connect to the internet:

• Stub AS: A stub AS has only one connection to the internet through a single upstream provider. Most small businesses and organizations fall into this category. They depend entirely on their ISP for internet connectivity. If that connection goes down, they're offline.
• Multihomed AS: A multihomed AS connects to the internet through multiple providers for redundancy. If one connection fails, traffic can flow through another. This is common for medium to large organizations that can't afford downtime. They pay multiple ISPs but don't carry transit traffic for others.
• Transit AS: A transit AS not only connects to the internet but also carries traffic between other networks. These are your ISPs and backbone providers. They have customers who depend on them for connectivity and often have peering relationships with other transit providers.