What is ASN? What is an autonomous system?

ASN stands for "Autonomous System Number". This is a unique identification number assigned to an autonomous system (AS) on the Internet. An ASN makes it possible to uniquely identify a network worldwide. Without these numbers, the Internet could not function - currently. They are an essential building block to ensure security, efficiency, and scalability of the global network we call the Internet.

What is an autonomous system (AS)?

An autonomous system can be understood as a network consisting of a certain number of IP networks, routers, and similar devices. An AS is typically managed by a specific entity, such as a company or an ISP. An AS is capable of receiving, processing, and forwarding IP packets. It acts autonomously - meaning it decides how these data packets are handled within its own network. It follows its own routing policy. Metaphorically speaking, you can imagine an AS as an independent transport company that transports packages from A to B and decides which route to take. At the same time, it can also cooperate with other transport companies to transport packages to distant locations.

Who operates autonomous systems (AS)?

• ISPs or Internet Service Providers: This group offers services in connection with Internet (DSL, vDSL, fiber optic, ...). In Germany, Deutsche Telekom or Vodafone would be examples.
• Corporations: Large corporations like Uber, Google, or Amazon operate their own autonomous systems to provide their services. In Germany, Helios Clinics, Vattenfall, or TAZ operate their own AS.
• Universities: Educational institutions, such as universities, also operate their own autonomous systems to support their research and teaching - Yale University would be an example of this. In Germany, some institutes of the Helmholtz Association have their own AS. The University of Stuttgart also operates its own AS.
• Government agencies: Not directly visible in Germany at least, but government agencies also operate their own autonomous systems to provide their services.

What is the function of ASNs?

Without ASNs, routing (or simply explained, communication) on the Internet doesn't work. Certain routers use ASNs to transmit data packets with high efficiency. In this context, "BGP" (Border Gateway Protocol) plays an important role. BGP is a routing protocol that connects the various autonomous systems. This allows determining the best path for a data packet. ASNs are generally often used in these ways:

1. ASNs are used to determine the path a packet should take to reach its destination as quickly as possible.
2. Routers use ASNs to make decisions based on an AS's routing policy.
3. The traceability of ASNs also allows preventing routing problems/loops.

Generally, a distinction is made between public and private ASNs:
IANA (Internet Assigned Numbers Authority) and RIRs (Regional Internet Registries) assign and manage public ASNs. They serve communication within the public Internet.
Private ASNs, on the other hand, are used for internal purposes and are not publicly accessible. They are typically used in private networks to enable communication between different locations.

What number ranges and representation types exist for ASNs?

Initially, there were only 2-byte or 16-bit ASNs. However, this led to an availability problem over time, as 16 bits only allows for 65,535 ASNs. 4-byte or 32-bit ASNs were therefore introduced. These allow for 4,294,967,296 ASNs. ASNs are generally displayed in the format "AS" followed by a number, for example "AS65000". Specifically for 32-bit ASNs, you occasionally see the following notation: AS65000.1. In this case, it's a 32-bit ASN split into two parts.

What requirements must be met to obtain an ASN?

Getting your own ASN is not easy. You must prove that this is required or necessary because, for example, you need to fulfill your own routing policy (routing ruleset). Certain guidelines must also be permanently met - these are provided and controlled by the responsible RIR. For the ASN to be globally accessible, there must also be at least two BGP peering partners that announce the ASN.

What role do IP addresses play in AS?

Each AS receives and manages its own IP address ranges (for example, 141.80.0.0/16). These indicate which IP addresses belong to it and can be used. In the example "141.80.0.0/16", this is a /16 network that encompasses 65,536 IP addresses. The operator of the AS can assign each of these IP addresses to one or more devices, networks, servers. These are then accessible on the Internet via these IP addresses.

What role does BGP play in AS?

The Border Gateway Protocol serves to exchange certain information (routing information) between different autonomous systems. BGP is what's known as a "path vector protocol". It uses vectors to determine the best path for a data packet through multiple ASes.

What role do Routing Policies play in AS?

Routing Policies are rules that determine how data packets are handled within an AS. The controlling AS instance can use the routing policy to determine, for example, that packets of a certain origin are prioritized or not prioritized. It can also be determined that packets from a certain country must take a certain route. In sum, this ruleset influences how data traffic flows within the AS.