Article Summary: This article explains what an Autonomous System Number is, how ASNs are assigned by Regional Internet Registries, how BGP uses ASNs to route internet traffic, and what information an ASN lookup reveals. It covers the five RIRs, the distinction between ASNs and IP ranges, and practical use cases for ASN lookups in network engineering, security research, and infrastructure management.
What Is ASN Lookup?
An ASN lookup retrieves all publicly available information about an Autonomous System Number (ASN) — the globally unique identifier assigned to a network that participates independently in internet routing. Enter any ASN in the format AS15169 or simply 15169, and the tool returns the network's official name, description, country of registration, assigned RIR, and the number of IPv4 and IPv6 prefixes it announces to the global BGP routing table.
ASN data is public by design — it is published in the global BGP table and in each RIR's WHOIS database. Every ISP, cloud provider, CDN, university, and large enterprise that manages its own internet routing has at least one ASN. Understanding ASNs is fundamental to understanding how the internet's routing infrastructure works at a macro level.
How It Works
What Is an Autonomous System?
An Autonomous System (AS) is a collection of IP networks and routers under the control of a single organization that presents a unified routing policy to the internet. "Autonomous" means the network makes its own routing decisions internally and exchanges traffic with other autonomous systems at peering points. The internet is, at its core, a network of tens of thousands of autonomous systems exchanging BGP routing advertisements with each other.
Each AS is identified by its unique ASN. Original ASNs are 16-bit numbers (1–65535), giving 65,535 possible values. Due to internet growth, 32-bit ASNs were introduced (up to 4,294,967,295), dramatically expanding the available pool. AS numbers in the range 64512–65535 and 4200000000–4294967294 are reserved for private use — similar to RFC 1918 private IP addresses — and should never appear in the global routing table.
Border Gateway Protocol (BGP)
BGP (Border Gateway Protocol) is the routing protocol that binds the internet together. Each autonomous system uses BGP to announce its IP prefixes to neighboring ASes, and to learn the prefixes announced by others. BGP uses ASNs as path identifiers — every BGP route advertisement carries the full list of ASNs it has traversed (the AS path), which allows routers to detect routing loops and select optimal paths. Without ASNs, the internet's distributed routing system could not function.
IP Prefix Announcements
An ASN lookup reveals how many IPv4 and IPv6 prefixes the autonomous system announces. A prefix is a block of IP addresses (e.g., 8.8.8.0/24) that the AS has told the global BGP table it is responsible for routing. Large ISPs and cloud providers announce hundreds or thousands of prefixes. Smaller organizations may announce a single prefix covering their allocated IP block.
Common Use Cases
Network Engineering and Peering
Network engineers use ASN lookups to plan peering arrangements — agreements between autonomous systems to exchange traffic directly rather than through paid transit providers. Before initiating peering, engineers verify an ASN's prefix count, traffic volume, and peering policy. Large ASNs with high prefix counts and open peering policies (like many major IXP participants) are attractive peering partners.
Security Research and Threat Intelligence
Security teams use ASN lookups to contextualize attack traffic. An IP associated with a known bulletproof hosting ASN — one that tolerates or ignores abuse complaints — is a strong indicator that traffic from that IP is malicious. Many threat intelligence feeds are organized by ASN, allowing security teams to block entire autonomous systems known to originate spam, port scans, or DDoS traffic.
IP Block Attribution
ASN lookup is the fastest way to determine which organization controls a given IP block. Combined with IP geolocation, it answers both "where is this IP?" and "whose network is this IP on?" — two distinct questions with distinct answers. A Google IP (ASN AS15169) can be in a data center in any country, but the ASN immediately identifies the controlling organization.
Technical Reference
| RIR | Region | Website | ASN Range (16-bit) |
|---|---|---|---|
| ARIN | North America | arin.net | 1 – 1799, 2697 – 2918, and others |
| RIPE NCC | Europe, Middle East, Central Asia | ripe.net | 1877 – 2042, 5377 – 5631, and others |
| APNIC | Asia-Pacific | apnic.net | 4608 – 4864, 7467 – 7722, and others |
| LACNIC | Latin America and Caribbean | lacnic.net | 26592 – 26623, 27648 – 28671, and others |
| AFRINIC | Africa | afrinic.net | 36864 – 37887 and others |
Frequently Asked Questions
What is an ASN?
An ASN (Autonomous System Number) is a globally unique number assigned to a network — an autonomous system — that independently manages its own IP routing policy on the internet. ISPs, cloud providers, and large enterprises each have one or more ASNs. The ASN is used in BGP routing advertisements to identify which network is announcing which IP prefixes.
How are ASNs assigned?
ASNs are assigned by the five Regional Internet Registries (RIRs): ARIN (North America), RIPE NCC (Europe/Middle East/Central Asia), APNIC (Asia-Pacific), LACNIC (Latin America), and AFRINIC (Africa). Organizations apply to their regional RIR, demonstrating a technical and administrative need for an ASN — typically that they connect to two or more upstream providers (multihoming) and need to manage their own routing policy. The RIR verifies the application and assigns an ASN from its allocated pool.
What is BGP and how does it relate to ASNs?
BGP (Border Gateway Protocol) is the internet's inter-domain routing protocol — the system by which autonomous systems tell each other which IP prefixes they can route to. Every BGP route advertisement includes an AS path: the ordered list of ASNs the route has traversed. This path is how routers detect loops (an AS will not accept a route containing its own ASN) and how traffic engineering is performed. ASNs are the fundamental identifiers that make BGP routing possible.
Can a small company get its own ASN?
Yes, but it requires meeting the RIR's eligibility criteria. Most RIRs require the applicant to demonstrate that they are multihomed — connected to at least two different upstream ISPs — and that they have a legitimate need for independent routing policy. A company that connects to the internet through a single ISP does not need an ASN; that ISP's ASN covers their IP block. Annual fees apply for ASN maintenance with the RIR.
What is the difference between an ASN and an IP range?
An ASN identifies the network organization responsible for routing — it is the "who." An IP range (prefix) is the block of IP addresses being routed — it is the "what." One ASN can announce many IP prefixes, and in rare cases, IP prefixes can be transferred between ASNs. The ASN lookup tool returns both: the ASN's identity information and the count of IPv4/IPv6 prefixes it currently announces to the global BGP routing table.
Conclusion and Takeaways
The ASN lookup tool provides a direct window into the internet's routing infrastructure. By identifying the autonomous system behind any IP address or ASN number, it reveals the organization responsible for that network, the geographic region it serves, and the scale of its IP address announcements. Whether you are a network engineer planning peering arrangements, a security researcher profiling attack infrastructure, or a developer understanding where your traffic is going, ASN data is an essential layer of internet intelligence.
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