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Autonomous System Number

What is an Autonomous System Number?


An autonomous system number is a unique identifier that is available globally and allows the autonomous system to share routing data with other systems.


An autonomous system (AS) is a community of IP prefixes whose external routing policy is clearly defined. To be able to interact with multiple autonomous systems, each must have a unique identifier. Estimates of autonomous systems may be public or private. Meanwhile for systems to share information over the Internet, public ASNs are required. Alternatively, a private ASN may be used when a device communicates with a single provider via the Border Gateway Protocol (BGP).

Regional Internet registries

The Internet Assigned Numbers Authority (IANA) is responsible for coordinating DNS Root, IP routing, and other services for the Internet Protocol, including ASNs, globally. IANA assigns ASNs to Regional Internet Registries (RIRs), which are organizations controlling Internet number services in a given region of the world.

The five regional Internet registries are:

  • African Network Information Center (AFRINIC)
  • American Registry for Internet Numbers (ARIN)
  • Asia-Pacific Network Information Centre (APNIC)
  • Latin American and Caribbean Network Information Centre (LACNIC)
  • Réseaux IP Européens Network Coordination Centre (RIPE NCC)

Moreover, an unincorporated group called the Number Resource Group unites the five RIRs. The mission of the NRO is to contribute to an open, safe and efficient Internet through the coordination of joint RIR activities and initiatives, such as Resource Certification (RPKI) and Internet Governance activities.

How Autonomous System Numbers Work

If your autonomous system needs an Internet connection or a separate network connection, it will need to use the Border Gateway Protocol which requires manual configuration. One of the configuration options is to define the other autonomous systems you create a link with. The only way to identify an autonomous system is via their ASN.

Obtaining an autonomous system number

Obtaining an autonomous number of the network, as mentioned, requires going through one of the five regional Internet registries. For example, we’ll cover how to obtain an autonomous system number from the American Internet Numbers Registry.
First, you must qualify for an ASN by either having a unique routing policy or a multihomed site.

  • Multihomed certification requires you to provide the external gateway protocol to be used; the IP addresses used on your network. Morever, the name of all upstream providers and peers, together with at least two contract verifications.
  • Unique routing qualification requires proof that your autonomous system’s routing policy differs from the routing policies of its peers.

If you meet one of the qualifications, you can submit an ASN request through ARIN Online. After approval, you will have to sign a Registration Services Agreement and pay a container fee for your ASN.

Autonomous system numbers at the edge

About the advantages of edge computing, autonomous device numbers are directly tied to Internet exchange points. Every network using peering at an IXP is an autonomous device with an autonomous number. Peering. BGP is only possible because each AS has a unique identifier and is configured manually to speak directly to other unique identifiers.

Examples of Autonomous System Numbers

There are four types of autonomous systems that generally need an ASN. These include:

  • Multihomed – Connected to more than one autonomous system.
  • Stub – Only connected to one other autonomous system.
  • Transit – Provides connections through itself. For example, network A can connect to network C directly or by crossing over network B.
  • Internet Exchange Point – Autonomous system created by the physical infrastructure located at Internet exchange points.

Autonomous system number formats

Before 2007, all 2-byte, or16-bit, autonomous machine numbers are use. This has issued IANA 65.536 distributable ASNs. This number has always  bound to run out, equivalent to IPv4 addresses. Just as IPv6 was develope, four-byte (32-bit) ASNs were create to remedy the problem. The new system includes autonomous machine numbers 4.294.967.296.
With the switch to 4-byte, people grew concerned that number representation would become too difficult. To mitigate those concerns, two alternative ways to represent the number were create.

  • The standard method for displaying the number is call asplain, which is a simple decimal representation.
  • The asdot+ method breaks the number into low and high-order 16-bit values and separates them by a dot. For example, 65525 would be display as 0.65525, 65537 would be display as 1.0, 65680 would be display as 1.144, and so on.
  • The asdot method is a mixture of asplain and asdot+. Such as, any number that is in the 2-byte range is display in asplain format, 65525 would be 65525. Any number that is outside of that range is display in asdot+ format, 65680 would be 1.144.

Key Takeaways

  1. An autonomous system number is necessary for any interconnection between two autonomous systems, such as peered networks at Internet exchange points.
  2. There are five regional Internet registries through which you can apply for an ASN if you meet the necessary requirements.

Autonomous System Number
Autonomous System Number