What is an Autonomous System

The internet is a complex web of networks. Each network is an autonomous system. All connected by border gateway protocol.

To understand how the backbone of the internet works, we need to understand autonomous networks.

Here, we will discuss what is an autonomous system and how this relates to the internet.

What is an Autonomous System

An autonomous system is a collection of network prefixes within the internet. A collection of network prefixes are grouped into an autonomous system (AS). An AS is an individual network on the internet. Each network, or AS, has an autonomous system number (ASN).

To understand a bit further, we can explain where an AS fits in the composition of the internet. The internet comprises internet protocols (IPs), network prefixes and autonomous systems. IPs are grouped into network prefixes and network prefixes are grouped into autonomous systems. Autonomous systems are then owned by internet service providers (ISPs).

In the early days of the internet, an AS was only able to be owned by one entity. For example, one ISP would own one AS. This is no longer the case. An ISP can own multiple AS’s and they will use border gateway protocol (BGP) to communicate between them.

Autonomous systems communicate between each other through BGP. For one AS to communicate to another, they identify with autonomous system numbers.

What is an Autonomous System Number

Autonomous system numbers (ASNs) are the unique identifiers that are given to an AS. The identifier allows an AS to communicate both externally and internally. An AS can have a private ASN and a public ASN.

Private ASN: A private ASN is a unique identifier for an AS that is used for communicating an AS to one entity.

Public ASN: A public ASN is a unique identifier that is advertised to the public internet. This is also used to communicate between ASNs.

There is not a significant difference between public and private ASNs. As an ASN determines the next hop in BGP, whether public or private, the network route will need to be configured in the network router.

Autonomous systems are the foundation for BGP to communicate and route traffic through the internet.

The Autonomous System and Border Gateway Protocol Connection

Border gateway protocol is how an AS communicates with another AS. Also, BGP provides the information and algorithm to select the path data will take through the internet.

BGP uses the information in BGP routing tables to determine the next AS hop, which determines the route to its destination.

By default, BGP will send traffic the shortest logical distance. This means the fewest number of AS hops. This process is BGP best path selection or AS path selection.

BGP Best Path Selection

BGP best path selection governs how all internet traffic routes through the internet.

We discuss in detail how BGP best path selection works in our BGP best path selection post.

In summary:

BGP best path selection provides weights to various network paths on the internet. There are preferences given to locality and peering agreements. Also, the path length is considered.

This system is how an AS connects to another AS and routes your data through the internet.

Understanding Autonomous Systems

Autonomous systems have a few interworking’s which make it a bit easier to understand.

Autonomous systems have the interior gateway protocol and border gateway protocol.

Interior gateway protocol (IGP) communicates within an AS. Border Gateway Protocol communicates between AS’s. IGP routes data within an AS, where BGP routes data from one AS to another.

An AS will exchange data at specific locations as well. An internet exchange point (IXP) is where an AS exchanges data.

The IXP is where ISPs who own an AS exchange with other ISPs through peering agreements. Then, BGP will determine the shortest AS path and route data through the internet.

Autonomous systems are foundational to the internet. An AS is a network on the internet and BGP is how these networks communicate.

To learn more about networking terms as they relate to DevOps, you can download the DevOps Networking eBook.

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