An autonomous system number is a unique identifier that is globally available and allows its autonomous system to exchange routing information with other systems.
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).
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:
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.
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.
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.
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 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.
There are four types of autonomous systems that generally need an ASN. These include:
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.