What Is ASN? Introduction To Autonomous System Numbers

4 min read
21 October 2021
Mindaugas Kubilius

Learn the basics about autonomous systems and autonomous system numbers. Discover how to request an ASN.

What is autonomous system number.

An autonomous system number (ASN) is a unique identifier assigned to every autonomous system (AS). An AS is a group of networks connected to the internet, with a single routing policy.

Governmental organizations, internet service providers (ISP) or educational institutions manage autonomous systems. And the Internet Assigned Numbers Authority (IANA) is responsible for managing ASNs.

Do not worry if this explanation is a little confusing because we unpack what all of this means in our article. If you need more information, continue reading.

How do autonomous systems work?

If IP addresses are the postal addresses of the internet, an autonomous system acts as the town sorting office. It is responsible for the routing of external traffic to and from other systems. 

Data packets sent across the internet will go from one autonomous system to another until they find the system that contains the IP address to which they are sent. It is then down to routers within the autonomous system to route the packet to the destination IP address. 

This process of sending packets between systems is known as Border Gateway Protocol (BGP) routing. Autonomous systems share their routing policies with routers and other systems. Effectively, ASNs allow identifying the correct system, like postal addresses. 

How external and internal BGP work on different autonomous systems.
How external and internal BGP work

To be recognized as a distinct autonomous system, a network must have a unique routing policy. A routing policy contains a list of all the IP address ranges within the system (i.e., the IP address space) and a list of all other autonomous systems it connects to.   

The point of using ASNs is to increase the speed with which data moves around the internet. By giving each autonomous system an identifying number, BGP routing becomes much faster and more efficient. 

Without ASNs, there’s a risk that data packets would get lost on their way to their destination IP addresses, or take too long to arrive, making them useless. Because of this, ASNs are an integral part of the internet operating on a global scale. 

Different types of autonomous systems

There are three different types of autonomous systems:

  • Multihomed AS 
  • Stub AS 
  • Transit AS 

Multihomed AS has two or more connected systems and acts as a support mechanism keeping the internet connection uninterrupted if one AS fails.

Stub AS, also known as single-homed, is a type of AS that only connects to one other AS. However, it might have other private connections that the rest of the internet cannot see.

Transit A works as a waystation, allowing the data from other systems to pass through it, even if it comes from other, unassociated networks.

Different autonomous system numbers

IANA assigns two different formats of ASNs to networks: 2-byte and 4-byte.

There are two different types of autonomous system numbers because there are two different types of Internet Protocol addresses, IPv4 and IPv6. As more networks exist on the global internet, the demand for ASNs has increased, just like the demand for IP addresses. 

2-byte and 4-byte autonomous system number volumes.
2-byte vs. 4-byte autonomous system number volumes

The 2-byte/16-bit format was the only ASN format until 2007. However, since it allows for only 65,536 autonomous system numbers (0 to 65535), the Internet Assigned Numbers Authority decided to introduce the 4-byte format. 

First introduced in 2007, the 4-byte/32-bit autonomous system number format greatly increased the number of ASNs available to network operators. With 4 bytes, there are 4,294,967,296 ASNs (0 to 4294967295). 

This greatly increases the possible number of networks that can get ASNs. In theory, the 4-byte ASNs should satisfy global demand for the foreseeable future. 

Of the 2-byte ASNs, IANA reserves 1,023 (the 64512-65534 block) for private use. Of the 4-byte ASNs, it reserves 94,967,295 numbers (the 4200000000-4294967294 block) for private use. 

Obtaining an ASN

While IANA is responsible for assigning all ASNs, anyone hoping to obtain an ASN must apply to the relevant Regional Internet Registry. RIRs are organizations that manage internet resources and internet number assignments for specific areas of the world. 

Five Regional Internet Registries are responsible for five unique service regions:

Regional internet registry regions.
Service regions of five Regional Internet Registries

When an organization requests an ASN from one of these regional registries, it must demonstrate that it has either a unique routing policy or a multihomed site

To qualify under the basis of having a unique routing policy, network operators need more than just a clearly defined routing policy. They need to prove that their exterior gateway protocol differs from the routing policy of other networks and autonomous systems. 

Qualifying under the basis of having a multi-homed website is a little more complicated. You need to provide the BGP and IP addresses used on the network, as well as the ASN of upstream providers and contractual verifications.

If the applicant’s ASN request qualifies, they need to visit the website of their RIR and submit their routing information. Once they sign a Registration Services Agreement and pay a container fee, they get their ASN. 

Is it possible to obtain an ASN without dealing with an RIR directly? You can use managed LIR (Local Internet Registry) services. IPXO is a registered LIR at RIPE NCC, and we are ready for ASN allocations. Our ASN is 834. 


To quickly recap, an autonomous system number, ASN for short, is a number that identifies an AS. Businesses, educational establishments and governmental organizations use autonomous systems (groups of networks) to speed up the routing of information packets across the internet. 

An autonomous system number facilitates Border Gateway Protocol routing, in which data packets pass from one AS to another. If you need an ASN, you need to contact your Regional Internet Registry and submit proof of eligibility.

About the author

Mindaugas Kubilius

Network Administrator

Mindaugas is a Network Administrator at IPXO with more than 15 years of experience in the IT field. He specializes in building and maintaining various network infrastructures, as well as presenting top-notch engineering solutions to the public. After work, Mindaugas spends his time in nature.
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