ASN Lookup

What is an Autonomous System Number?

An Autonomous System Number (ASN) is a globally unique identifier that allows a network to participate in inter-domain routing on the internet. The internet is not a single monolithic network but a collection of tens of thousands of independently operated networks called Autonomous Systems (ASes). Each AS uses the Border Gateway Protocol (BGP) to advertise the IP prefixes it controls and to learn about routes to destinations in other ASes. Without ASNs, BGP would have no way to identify the origin of each route announcement, making the routing system unmanageable.

ASNs are 16-bit numbers (originally) ranging from 1 to 64511 for public use, with 64512 through 65535 reserved for private use. In 2007, the 32-bit ASN format was introduced to handle the growing demand, extending the range to 4,294,967,295. These are often written in "dot" notation: ASN 131072 is written as AS0.65536 or simply expressed as a plain decimal number. Most modern networks use 32-bit ASNs, though many legacy networks still operate with 16-bit numbers.

The five Regional Internet Registries (RIRs) manage ASN allocation for their respective geographic regions. ARIN covers North America, RIPE NCC covers Europe and the Middle East, APNIC covers Asia-Pacific, LACNIC covers Latin America and the Caribbean, and AFRINIC covers Africa. Each RIR maintains a publicly accessible WHOIS database that maps IP prefixes to their originating ASN and provides contact information for the network operator. This lookup tool queries these databases to return the ASN associated with any IP address.

How ASN data is used in practice

Network engineers rely on ASN data for BGP peering configuration, route filtering, and traffic engineering. When an organization needs to announce its IP prefixes to the internet, it must first obtain an ASN from its regional registry, then establish BGP sessions with its upstream transit providers or settlement-free peering partners. The ASN acts as the origin identifier in BGP path attributes, allowing each router along the path to know which AS originated each route prefix.

Security analysts use ASN data as a threat intelligence signal. When suspicious traffic is detected, looking up the source IP's ASN can reveal whether the attack originates from a known malicious hosting provider, a residential ISP, or a cloud platform. Many security orchestration tools automatically enrich IP addresses with ASN metadata to prioritize incident response. In DDoS mitigation, ASN information helps distinguish between legitimate traffic from a major CDN and attack traffic from a compromised network segment.

For internet researchers and policymakers, ASN data is essential for studying the structure and evolution of the internet. The number of allocated ASNs has grown from a few hundred in the early 1990s to over 100,000 today. Mapping ASN relationships reveals the internet's topology — which networks are tier 1 transit providers, which are content delivery networks, and which serve specific geographic regions. This analysis informs decisions about internet governance, interconnection policy, and infrastructure investment.

When to use ASN lookup

System administrators troubleshooting network connectivity issues can use ASN lookup to verify that their IP prefixes are correctly registered and announced. If a BGP session is not establishing, checking the ASN configuration is one of the first diagnostic steps. Security teams investigating suspicious login attempts or web scraping activity can identify the network provider behind the source IP address and contact their abuse department with the ASN ownership information.

Content providers optimizing delivery performance can use ASN data to identify which networks their users are connecting from and whether additional peering or CDN points-of-presence are needed in specific regions. The ASN-to-organization mapping also helps in deciding which traffic should be routed through which transit provider based on cost, latency, and capacity requirements.

Frequently asked questions