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Definition

An autonomous system number is a unique identifier that is globally available and allows its autonomous system to exchange routing information with other systems.

Overview

An autonomous system (AS) is a group of IP prefixes with a clearly defined external routing policy. In order for multiple autonomous systems to interact, each needs to have a unique identifier. Autonomous system numbers can be public or private. Public ASNs are required for systems to exchange information over the Internet. A private ASN can be used instead if a system is communicating solely with a single provider via Border Gateway Protocol (BGP).

Regional Internet registries

The Internet Assigned Numbers Authority (IANA) is responsible for globally coordinating DNS Root, IP addressing, and other Internet protocol resources, including ASNs. IANA assigns ASNs to regional Internet registries (RIRs), which are organizations that manage Internet number resources in a particular 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)

The five RIRs are united by an unincorporated organization called the Number Resource Organization. The NRO’s mission is to contribute to an open, stable, and secure Internet by coordinating joint RIR activities and projects, such as Resource Certification (RPKI) and Internet governance activities.

How Autonomous System Numbers Work

If your autonomous system requires an Internet connection or a connection to a separate network, it needs to use Border Gateway Protocol, which requires manual configuration. One of the configuration options involves identifying the other autonomous systems with which you are forming a connection. The only way that an autonomous system can be identified is via their ASN.

Obtaining an autonomous system number

As discussed, obtaining an autonomous system number requires going through one of the five regional Internet registries. As an example, we will cover how to obtain an autonomous system number from the American Registry for Internet Numbers.

First, you must qualify for an ASN by either having a unique routing policy or a multihomed site.

  • Multihomed qualification requires that you provide the exterior gateway protocol that will be used; the IP addresses that are in use on your network; and the ASN and name of all upstream providers and peers, along with at least two contractual 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

Autonomous system numbers are directly tied to internet exchange points when it comes to the benefits of edge computing. Each network that uses peering at an IXP is an autonomous system that has an autonomous system number. Peering through BGP is only possible because each AS has a unique identifier and is manually configured to talk 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

Until 2007, all autonomous system numbers were 2-byte, or 16-bit, numbers. This gave IANA 65,536 possible ASNs to distribute. This amount was always destined to run out, much like IPv4 addresses. Just like the creation of IPv6, 4-byte (32-bit) ASNs were created to remedy the issue. The new system provides 4,294,967,296 autonomous system numbers.

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 created.

  • The standard method for displaying the number is called 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 displayed as 0.65525, 65537 would be displayed as 1.0, 65680 would be displayed as 1.144, and so on.
  • The asdot method is a mixture of asplain and asdot+. Any number that is in the 2-byte range is displayed in asplain format, 65525 would be 65525; any number that is outside that range is displayed in asdot+ format, 65680 would be 1.144.

Key Takeaways

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