difference between as1 as2 and as3

AS1 vs AS2 vs AS3: The Ultimate Comparison

By William WhiteDecember 17, 2025

In the realm of Autonomous Systems (AS), the numbers 1, 2, and 3 are often seen, each signifying unique aspects that distinguish them from one another. AS1, AS2, and AS3 are not simply sequential numbers but each represents a distinct tier in the hierarchy of autonomous systems, with specific definitions and roles in the internet's infrastructure. Let's delve into the differences between these AS numbers to better understand their significance.

Before we dive into the details, let's clarify that an Autonomous System (AS) is a set of connected IP prefixes run by one or more network operators. It's a unit of rout political organization within the Internet routing system, identified using a unique AS number.

Understanding AS Numbers

AS numbers are crucial in the functioning of the internet as they enable routers to exchange routing information. They help in making the network routing globally interoperable. AS1, AS2, and AS3 were among the initial Autonomous Systems that played pioneering roles in shaping the internet's topology.

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AS numbers are a finite resource, much like IP addresses, and are managed by the Internet Assigned Numbers Authority (IANA). AS numbers are 16 bits long and can, thus, range from 0 to 65535. They are divided into various ranges for different purposes:

Private AS Numbers

Ranges 64512 - 65535 and 4200000000 - 4294967294 are reserved for Private AS Numbers. These are used within a private network and are not globally routed.

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These private AS numbers, especially those in the 64512 to 65535 range, are often used in network simulations or training environments to avoid conflicts with globally unique AS numbers.

Public AS Numbers

Ranges 1 - 65534 are for Public AS Numbers, which are globally unique and are used to identify Autonomous Systems on the global routing table. Here's where AS1, AS2, and AS3 reside.

The AS number was formally defined in 1993 with RFC 1058. At that time, the AS number space was far from exhausted, but with the exponential growth of the internet, the need for more AS numbers has arisen, leading to the creation of 32-bit AS numbers in 2000 with RFC 2375.

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AS1, AS2, and AS3: Pioneers in Internet History

AS1, AS2, and AS3 were among the first Autonomous Systems defined. They were assigned to early internet service providers (ISPs) and played crucial roles in the internet's infancy.

The first AS number, AS1, was assigned to the University of Southern California's Information Sciences Institute (ISI). ISI was a pioneer in internet research and played a crucial role in the development of the ARPANET into the internet we know today. AS2 was assigned to Stanford Research Institute (SRI), while AS3 went to the University College London (UCL).

AS1: A Leap into the Unknown

AS1 was a bold step into a new, uncharted territory of global networking.ISI, with its AS1, facilitated the transition from ARPANET's decentralized packet switches to modern TCP/IP-based routers. It was a pivotal moment in the evolution of the internet, marking the starting point of the AS numbering system.

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By today's standards, AS1 is relatively small, but its significance lies in its historical context. It stands as a testament to the pioneering spirit that has defined the internet's development and its continuous evolution.

AS2 and AS3: Early Builders of Internet Infrastructure

Following AS1, AS2 and AS3 were assigned to SRI and UCL, respectively. These institutions were also instrumental in shaping the early internet infrastructure. SRI, with AS2, played a significant role in the development of email and the Domain Name System (DNS), while UCL, with AS3, contributed to the development of key internet protocols like SNMP and BGP.

While AS2 and AS3 no longer exist in their original forms, their legacies live on in the internet protocols and standards they helped shape. Today, the AS numbering system and the Autonomous System concept are fundamental to the internet's global reach and interoperability.

From the humble beginnings of AS1, AS2, and AS3, the internet has grown into a complex web of global Autonomous Systems. As we continue to explore the vast terrain of the internet's infrastructure, it's essential to understand and appreciate the critical role these early AS numbers played in shaping the world we rely on daily.