ASN

An Autonomous System Number (ASN) is a unique identifier assigned to an Autonomous System (AS) on the internet. An Autonomous System is essentially a large collection of IP networks and routers under the control of a single organization that presents a common, clearly defined routing policy to the internet. Think of it as a “network passport” that allows a group of networks – whether belonging to an ISP, a university, a cloud provider, or a large enterprise – to announce its reachability and exchange routing information with other autonomous systems.

Without ASNs, the global routing table that powers the internet would be unable to distinguish between different administrative domains. They are the fundamental building blocks of inter-domain routing, primarily used by the Border Gateway Protocol (BGP), the protocol that glues the internet together.

The concept of autonomous systems emerged in the late 1980s as the internet transitioned from a research network to a global infrastructure. When BGP was introduced in 1989 (RFC 1105, later evolving through BGP-4 in 1994 with RFC 1771), it needed a way to identify independent routing domains. The original ASN space used 16-bit numbers, providing 65,536 possible values (0–65535).

Early allocations were conservative; AS 1 was assigned to the U.S. Department of Defense, and numbers grew slowly through the 1990s as commercial ISPs proliferated. By the mid-2000s, the rapid growth of the internet began exhausting the 16-bit space. In response, the IETF standardized 32-bit ASNs in 2007 (RFC 4893, later RFC 6793), expanding the pool to over 4 billion numbers.

The transition period allowed networks to represent 32-bit ASNs in “asdot” notation (e.g., 1.65546) for compatibility. By 2010, most modern routers and software fully supported native 32-bit ASNs. As of 2026, the vast majority of new allocations are 32-bit, though many legacy 16-bit ASNs remain in active use by major players.

ASNs enable the internet’s decentralized routing system by serving as unique labels in BGP updates. When a network wants to reach destinations in another administrative domain, routers exchange path information that includes a sequence of ASNs – the AS path – describing the route traffic will take.

A network announces its prefixes (blocks of IP addresses) to its BGP peers along with its own ASN. For example, when Cloudflare (AS13335) advertises its IP space, neighboring ASNs learn that to reach those prefixes, traffic should transit through AS13335. Peers then propagate this information further, prepending their own ASN to the path.

This creates an AS path attribute like: 3356 13335 (where AS3356 is Level 3 / Lumen, a common transit provider). Routers use the AS path for loop prevention (discarding routes containing their own ASN) and for policy decisions – preferring shorter paths or avoiding certain ASNs based on business relationships.

Example BGP Announcement:
Prefix: 104.16.0.0/12
AS Path: 3356 13335
Origin: IGP
Next Hop: 198.32.125.10

Autonomous systems interact in three main ways:

• Transit: A provider carries traffic for a customer to the rest of the internet (paid relationship).
• Peering: Two networks exchange traffic directly for mutual benefit, usually settlement-free.
• Customer-to-Provider: Downstream networks pay upstream providers for global reachability.

These relationships heavily influence routing policies and path selection.

Public ASNs are globally unique and visible in the global routing table. Private ASNs (64512–65534 in 16-bit, and large reserved 32-bit ranges) are used internally – for example, when a company connects to multiple providers (multi-homing) but removes the private ASN before advertising routes externally.

Originally all ASNs were 16-bit, but since 2007 the standard has shifted to 32-bit. As of 2026:

Allocation is handled regionally by the five RIRs under policies coordinated by IANA.

The allocation of ASNs is managed by the Internet Assigned Numbers Authority (IANA), which delegates blocks to the five Regional Internet Registries (RIRs): AFRINIC (Africa), APNIC (Asia-Pacific), ARIN (North America), LACNIC (Latin America), and RIPE NCC (Europe, Middle East, Central Asia).

Some of the most influential autonomous systems in 2026 include large transit providers and content networks such as Level 3 / Lumen (AS3356), Cogent (AS174), Hurricane Electric (AS6939), Cloudflare (AS13335), Google (AS15169), and Akamai (multiple ASNs). These networks often appear near the top of global routing tables due to their extensive peering and customer bases.

Organizations request an ASN when they need to implement sophisticated routing policies – most commonly for multi-homing to multiple ISPs for redundancy and load balancing. Enterprises, content providers, and cloud operators use ASNs to control traffic ingress and egress, optimize performance, and implement security policies such as filtering unwanted paths.

ASNs also enable traffic engineering: selectively announcing prefixes to influence inbound routing, or using communities to signal preferences to upstream providers.

Early exhaustion of the 16-bit space forced a complex transition to 32-bit ASNs, and some legacy equipment still struggles with full support. BGP itself remains vulnerable to misconfigurations and hijacks, where malicious or mistaken announcements can reroute traffic through unintended ASNs.

Allocation policies require justification of need, which can be bureaucratic for smaller organizations. The sheer scale of the global routing table – over a million prefixes and hundreds of thousands of ASNs – strains router memory and convergence times.

In today’s internet, ASNs are central to advanced security mechanisms like Resource Public Key Infrastructure (RPKI), which cryptographically validates route origin to prevent hijacks. MANRS (Mutually Agreed Norms for Routing Security) initiatives encourage best practices among ASN operators.

With the rise of edge computing, CDNs, and anycast, large networks increasingly use multiple ASNs strategically. IPv6 adoption has been seamless for ASNs, as BGP handles both address families uniformly. As the internet grows more decentralized with satellite constellations and community networks, new ASN allocations continue steadily.

Autonomous System Numbers remain one of the internet’s most critical yet invisible components. They allow thousands of independent networks to cooperate in delivering packets across the globe while maintaining administrative control. From the early days of a 16-bit namespace to today’s vast 32-bit ecosystem, ASNs have evolved to support an increasingly complex and resilient internet infrastructure. While challenges around security and scalability persist, ongoing standardization and security improvements ensure they will remain foundational for years to come.

• Autonomous system (Internet) - Wikipedia
• RFC 4271 – A Border Gateway Protocol 4 (BGP-4)
• RFC 6793 – BGP Support for Four-Octet AS Number Space
• IANA AS Numbers Registry

Information compiled from Wikipedia, IETF RFCs, RIR statistics, BGP routing reports (e.g., BGPStream, Hurricane Electric BGP Toolkit), and industry resources up to 2026.

• Autonomous System on Wikipedia
• IANA AS Numbers Registry
• RFC 4271 – BGP-4
• Hurricane Electric BGP Toolkit