Understanding ASNs: The Essential Guide to Autonomous System Numbers 

An autonomous system (AS) is a network, or a collection of networks managed by a one entity, for example governmental organizations, internet service providers (ISP) or educational institutions.  

Every autonomous system is given a distinct identification number called an autonomous system number (ASN). The Internet Assigned Numbers Authority (IANA) is responsible for managing ASNs. Keep in mind that ASN is necessary for routing your IP address. 

How do autonomous systems work? 

Autonomous system delivers data packets across the Internet from one AS to another, until it reaches the one containing destination IP address.   

It is then down to routers within that AS to send data packets to the destination IP address. 

Autonomous systems have clearly defined routing policies that govern how these systems share routing information with each other.  

Routing policies dictate how routers offer connectivity to various definitions. They go beyond basic reachability and routing metrics and cover economics, security, availability, and operational management. 

From an organizational perspective, clearly outlined policies serve as the basis of aligning business needs with the implementation of networks that satisfy those needs.

Border Gateway Protocol 

The Border Gateway Protocol (BGP) allows exchanging routing information between autonomous systems and enables data routing.

When someone submits data via the Internet, BGP is responsible for picking the best route for that data to travel. 

ASNs are important in the context of BGP routing because it helps the protocol to choose the fastest route for data packets to their destination. 

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. 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 interconnects with multiple autonomous systems.  This kind of setup increases the As’s reliability and redundancy. Multihomed AS typically uses interconnections for its own traffic, rather than transit services between ASes. 

Stub AS, also known as single-homed AS, is a type of autonomous system that only interconnects with one other AS. It does not transit traffic between different ASes.  

Transit AS provides network access between two or more autonomous systems and allows networks to exchange traffic through it. Most large ISPs are transit ASes. 

2-Byte vs. 4-Byte ASNs: meeting growing network needs 

IANA assigns two different formats of ASNs to networks:

• 2-byte/16-bit 
• 4-byte/32-bit 

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

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 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) and of the 4-byte ASNs – 94,967,295 numbers (the 4200000000-4294967294 block) for private use.

ANSs in real life 

ISPs are perhaps the most common example of entities that operate autonomous systems.  

Each ISP typically has its own ASN, which is used to optimise data routing, support their internet infrastructure and improve reliability.  

Another example – Cloud service providers like Amazon Web Services (AWS), Microsoft Azure, and Google Cloud Platform (GCP). They operate massive networks of data centers and infrastructure globally.  

ASNs in their context help with efficiently managing the vast amount of traffic generated by cloud services. 

Getting your 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: 

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

The requirements to get and ASN from each of these regional registries may differ. Make sure to check them on their official websites.  

For example, if your regional registry is ARIN, their website states that “organizations may qualify for an ASN either by providing the names and ASNs of two upstream ISPs they’ll be using to multi-home, or by describing a unique routing policy.” 

To qualify for a unique routing policy, network operators need to prove that their external routing policy differs from the routing policy of other networks and autonomous systems.  

When qualifying under the basis of having a multi-homed website, you need to provide the exterior gateway protocol that will be used, 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 submit their request through ARIN online. Once they sign a Registration Services Agreement and pay a container fee, they get their ASN. 

Last year, RIRs assigned 5,942 ASNs in total.

Is it possible to obtain an ASN without dealing with 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 Sponsorship. Our ASN is 834.  

Conclusion

Understanding Autonomous System Numbers (ASNs) is key to navigating the complexities of internet infrastructure.  

ASNs act as unique identifiers for autonomous systems, facilitating efficient data routing and ensuring reliable communication across networks. 

Whether you’re ISPs, cloud service providers, or individual organizations, if you need an ASN, you need to contact your Regional Internet Registry and submit proof of eligibility. 

FAQ 

• What is an Autonomous System Number (ASN) and Autonomous System (AS)? 
  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. 
• Why is ASN important? 
  ASN helps routers find the best path for data to travel between networks, ensuring fast and reliable delivery. 
• What role do ASNs play in the Border Gateway Protocol (BGP)? 
  ASNs are used in the Border Gateway Protocol (BGP) to identify autonomous systems and determine the best path for data to travel across the Internet. BGP routers rely on ASNs to make routing decisions and optimize the flow of traffic between networks. 
• Who assigns ASNs, and how can organizations obtain them? 
  ASNs are assigned by regional Internet registries (RIRs) such as ARIN, RIPE NCC, and APNIC. Organizations can obtain ASNs by applying to their respective RIR and demonstrating their need for autonomous routing control.