What Is an ASN? Autonomous System Numbers Explained

What Is an ASN?

An Autonomous System Number (ASN) is a unique identifier assigned to a network or collection of networks that operate under a single routing policy. Think of an autonomous system (AS) as a large, independently managed network — such as an ISP, a cloud provider, a university, or a major enterprise. The ASN is the number that identifies that network to the rest of the internet.

For example, when you send an email or visit a website, your data travels across multiple autonomous systems before reaching its destination. Each AS uses its ASN to announce which IP address ranges it controls, so routers across the internet know where to forward traffic. Without ASNs, the global routing system would have no way to distinguish one network from another.

How ASNs Work: BGP Routing Basics

ASNs are the foundation of BGP (Border Gateway Protocol), the routing protocol that holds the internet together. BGP is how autonomous systems exchange reachability information — essentially telling each other which IP prefixes they can route traffic to.

Here is a simplified view of how BGP routing works:

1. A network operator (e.g., an ISP) is assigned an ASN and a block of IP addresses.
2. The operator configures their BGP routers to announce their IP prefixes along with their ASN.
3. Neighboring autonomous systems receive these announcements and propagate them further, building a global routing table.
4. When traffic needs to reach an IP address, routers use the BGP table to determine the best path through a series of ASNs — this is called the AS path.

You can look up the ASN for any IP address or network using our ASN Lookup tool.

Types of ASNs

ASNs come in several categories that are important to understand:

• 2-byte ASNs (16-bit): The original format, ranging from 1 to 65535. These were sufficient for decades but began running out as the internet grew.
• 4-byte ASNs (32-bit): Introduced via RFC 6793, ranging from 1 to 4294967295. All modern BGP implementations support 4-byte ASNs.
• Public ASNs: Globally unique numbers used for routing on the public internet. These are what most people refer to when discussing ASNs.
• Private ASNs: Reserved ranges (64512–65534 for 2-byte, 4200000000–4294967294 for 4-byte) used for internal routing between an organization and its upstream provider. Private ASNs are stripped before announcements reach the global routing table.

Who Assigns ASN Numbers?

ASN allocation follows a hierarchical system managed by global internet governance bodies:

• IANA (Internet Assigned Numbers Authority): Manages the global pool of ASNs and allocates blocks to Regional Internet Registries.
• RIRs (Regional Internet Registries): Five organizations distribute ASNs within their regions — ARIN (North America), RIPE NCC (Europe/Middle East), APNIC (Asia-Pacific), AFRINIC (Africa), and LACNIC (Latin America).
• LIRs/ISPs: Local Internet Registries and ISPs request ASNs from their RIR on behalf of organizations that need them.

To obtain an ASN, an organization must demonstrate a legitimate need for a unique routing policy — typically meaning they connect to multiple upstream providers (multihoming) or run a large network that requires independent routing control.

Why ASNs Matter for Email and Security

ASNs play a significant role in email deliverability and network security, even though most people never think about them:

• IP reputation by ASN: Email providers and anti-spam systems often evaluate sender reputation at the ASN level. If many IPs within the same ASN are sending spam, the entire network can suffer deliverability issues. You can check whether your sending IPs are listed on any blocklists using our Blacklist Checker.
• BGP hijacking detection: Attackers sometimes announce IP prefixes they do not own (BGP hijacking) to intercept or redirect traffic. Monitoring ASN announcements helps detect these attacks.
• Abuse tracking: Security researchers and abuse teams use ASN data to identify networks harboring malicious activity, botnets, or phishing infrastructure.
• Geolocation and compliance: ASN data reveals which organization operates a network and where it is located, which is valuable for compliance and IP geolocation analysis.

How to Look Up an ASN

Looking up ASN information is straightforward. You can query by IP address to find which ASN it belongs to, or search by ASN number to see which IP prefixes it announces. Our ASN Lookup tool lets you do both instantly, showing you the organization name, country, allocated IP ranges, and more.

For deeper investigation, you can combine ASN lookups with other tools:

• Reverse DNS Lookup — Find the hostname associated with an IP address.
• WHOIS Lookup — Get registration details for domains and IP ranges.
• DNS Lookup — Query DNS records for any domain.

Common ASNs You Should Know

Some ASNs appear frequently in network analysis and email infrastructure. Here are a few of the most well-known:

• AS15169 — Google: One of the largest ASNs on the internet, carrying traffic for Google Search, Gmail, YouTube, and Google Cloud.
• AS13335 — Cloudflare: Powers Cloudflare's global CDN and DNS infrastructure, including the popular 1.1.1.1 resolver.
• AS16509 — Amazon (AWS): Amazon Web Services hosts a significant portion of the internet's infrastructure, making this one of the most commonly seen ASNs.
• AS8075 — Microsoft: Covers Azure, Office 365, Outlook.com, and Microsoft's global network.
• AS32934 — Meta (Facebook): Handles traffic for Facebook, Instagram, WhatsApp, and Meta's infrastructure.
• AS36351 — SoftLayer (IBM Cloud): A major hosting provider frequently seen in email infrastructure analysis.

Knowing which ASN an IP belongs to helps you quickly identify whether email traffic is coming from a legitimate cloud provider, a known hosting company, or a suspicious network. Use our ASN Lookup tool to check any IP or ASN instantly.