ASN Lookup - WHOIS, BGP Routing & IP Range Details for Any ASN

Going from an AS number to its IP ranges is the same lookup read the other way. Enter the number and the result lists every IPv4 and IPv6 prefix the network announces, in CIDR notation, with the first and last address of each range worked out for you. Large networks run to thousands: AS3356 announces close to 4,000 prefixes and AS15169 around 1,400.

That list is what you need to allowlist or block a provider properly, to size a network before an audit, or to check whether an address you are investigating falls inside a range you already know. A map view plots where those ranges resolve to, which is a quick way to see whether a network is regional or spread across continents.

The Internet is built from thousands of smaller, independent networks called Autonomous Systems (AS). An Autonomous System (AS) is a collection of IP networks and routers controlled by a single entity (like a corporation, university, or government agency) that presents a common routing policy to the Internet. Think of an AS as a neighborhood in a city: it has its own internal IP addresses and routers managed by a single entity (like an ISP or a large company). The 'autonomous' part means the organization has full control over its internal traffic routing. The Autonomous System Number (ASN) is a globally unique identifier assigned to that AS. The ASN is essential for the exchange of routing information between different networks.

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.

Originally ASNs were 16-bit numbers, 1 to 65535. The internet outgrew that, so they are now 32-bit, running up to 4,294,967,295. A publicly routable AS number is required to take part in global routing.

Some ranges are set aside and will never appear in the global routing table, so a lookup on one returns nothing by design rather than by error. 64512 to 65534, and 4200000000 to 4294967294, are private - organizations use them inside their own networks. 65536 to 65551 is reserved for documentation and examples. AS0, AS65535 and AS4294967295 are reserved outright, and AS23456 is AS_TRANS, a placeholder a 16-bit router uses to stand in for a 32-bit number. The tool tells you which one you have entered.

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.

The Border Gateway Protocol (BGP) is the internet's core routing protocol. ASNs are the fundamental units BGP uses to exchange data. BGP routers use the ASN to determine the most efficient AS Path for routing traffic. When you perform an ASN lookup, you are essentially querying this BGP routing database to understand the network’s advertised routes.

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.