ASN

What an ASN Identifies

An Autonomous System Number (ASN) is a globally unique integer assigned by a Regional Internet Registry (RIR) — ARIN, RIPE NCC, APNIC, LACNIC, or AFRINIC — that identifies a single autonomous system (AS). An Autonomous System (AS)A large network or group of networks under a single administrative policy, identified by a unique Autonomous System Number (ASN). The internet is composed of tens of thousands of interconnected autonomous systems. is a collection of IP prefixes under common administrative control that presents a unified routing policy to the internet. Every major ISPInternet Service Provider. A company that provides internet access to consumers and businesses, assigning public IP addresses and routing traffic to the wider internet. Examples include Comcast, AT&T, and SK Broadband., cloud provider, university, and large enterprise that connects directly to the internet operates at least one ASN.

16-bit and 32-bit ASNs

The original ASN space was 16-bit, providing 65,536 numbers (0-65535). ASNs 64512-65534 are reserved for private use, analogous to Private IP AddressAn IP address from reserved ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) used within local networks. Private addresses are not routable on the public internet and require NAT for external communication. addresses — these appear in internal BGPBorder Gateway Protocol. The routing protocol that exchanges reachability information between autonomous systems, effectively determining how data traverses the internet. Often called the "postal service of the internet." configurations that are never advertised externally. As the 16-bit space neared exhaustion, RFC 4893 extended ASNs to 32 bits (4-byte AS), expanding the pool to over four billion. 32-bit ASNs are written in ASPLAIN format (a single decimal number, e.g., 131072) or ASDOT format (e.g., 2.0).

Lookup and Analysis

WHOIS Lookup reveals the ASN registered for any IP address, along with the AS name, RIR record, and announced prefixes. IP Lookup provides ASN context alongside geolocation data. Network engineers use ASN information to understand PeeringA mutual arrangement between two networks to exchange traffic directly and freely (settlement-free peering) at an interconnection point, bypassing third-party transit providers. Peering reduces costs and latency for both parties. relationships, identify TransitA paid arrangement where one network (the customer) pays another (the transit provider) for access to the rest of the internet. Unlike peering, transit provides full routing table access and is the primary way smaller networks connect to the global internet. providers, and attribute traffic sources. BGP looking glass servers operated by major networks allow querying which ASN path the global routing table shows to any given prefix — essential for diagnosing BGP HijackAn attack or misconfiguration where a network falsely announces ownership of IP prefixes it does not control, causing internet traffic to be rerouted through the attacker's network. Can be used for surveillance, traffic interception, or denial of service. incidents.