What are Autonomous System Numbers (ASN) on the Internet?

The Internet Assigned Numbers Authority (IANA) assigns every Autonomous System (AS) on the internet a 16-digit Autonomous System Number (ASN). . This identification number is what allows independent systems on the internet to both control the routing information within their own networks and exchange routing information with other ISPs. In order to understand the Autonomous System Numbers on the internet, let’s back up and talk about Autonomous Systems. An Autonomous System (AS) is a connected group of one or more IP prefixes that are run by one or more network operators with a single, clearly defined routing policy. These are necessary because the internet is not governed by one body that oversees all routing. Instead, there are large autonomous systems that need to connect to one another in order to communicate. Here are some AS examples you have likely heard of: As you can see, an AS could be a search engine, an institution, an ISP, or any other group of shared IP addresses. Each AS on the internet needs a unique ASN for several reasons. In short, ASNs are fundamental to BGP inter-domain routing. Let’s take a look at some common questions about Autonomous System Numbers At the same time that IANA oversees global coordination and allocation of ASNs, it also delegates regional distribution to several different Regional Internet Registries . These RIRs include: Organizations apply through their regional RIR to receive their ASN assignment. The RIR manages distribution and registration within its region, based on global policies established by ICANN and IANA. This hierarchical system allows decentralized regional administration at the same time that it coordinates unique ASN allocation globally. RIRs assign public ASNs as unique identifiers to allow each AS to be visible on the public internet. However, there are also private ASNs that are for internal use only. These numbers only have significance as part of their local network , and they do not need to be unique from all other global ASNs. The public internet can’t see these. Large companies will often use private ASNs internally, as they can aid management segmentation of a large network before announcing through a public ASN. Private ASNs also reduce the global BGP routing table size. That can lead to improved router performance, better scalability and stability, and increased convergence speed. Originally, all ASNs were 16-bit integers, allowing for 65,536 possible assignments. The internet obviously grew far beyond that limited number of ASes. So it was expanded into 32-bit integers in 2007. ASes connect to one another and exchange routing information using the Border Gateway Protocol (BGP). BGP allows each AS to control the routing structure within their own network, and it also provides an exchange route with every other AS. When an AS originates a route within its network, it appends its own ASN to the routing announcement. This provides ownership and path information when propagating routes. Here’s an example: If ASN 64496 originates a route to this subnet: 192.168.0.0/24. This is announced to its neighbors as “192.168.0.0/24, 64496.” (Neighbors are other ASes that 64496 has a direct BGP peering relationship with.) Now, this order of numbers and characters denotes that ASN 64496 is the originator – and owner – of the route. It provides authoritative origination information to other networks. BGP propagates this route to other ASes, and each AS adds its own ASN to the path. For example, if ASN 64500 receives the announcement from 64496, it would propagate it like this: “192.168.0.0/24, 64496 64500.” This process builds up an Autonomous System Path  (AS_PATH) that shows the sequence of each AS that the route has traversed. This method is how BGP uses ASNs to provide routing loop avoidance, policy control, and identification of where the route came from. A BGP peering session is a direct connection between routers in order to exchange routing information. This is how AS_PATHs are established. Some additional facts about these peering sessions: There are 3 kinds of ASes that will be identified via ASN. Several groups of people need to know about ASNs. For example, network engineers need to use them to configure and troubleshoot any issues related to BGP routing protocol. Employees of ISPs make extensive use of BGP and ASNs to interconnect and route traffic between networks. If you work at a web company like Facebook, Google, or Netflix, someone in your organization has to know about BGP and ASNs, because this is the system used to optimize connectivity and traffic delivery. Cloud providers operate their own ASNs and utilize BGP heavily to manage routing and interconnect their cloud regions. Corporate network administrators connect ISP ASNs and exchange routes via BGP. ASNs and the BGP form the backbone of internet routing. By assigning each independent network an ASN, traffic can be efficiently exchanged between domains while still allowing customized routing policies. The web as we know it today could not function without BGP enabling ASes to dynamically share reachability information. Whether you are emailing a colleague, streaming a video, or accessing cloud servers, your data is traversing multiple ASNs along its journey. The next time you browse online, consider the hidden world of ASes communicating and routing traffic to bring information to your fingertips. While the complexities of interdomain routing remain largely invisible to end users, it is this worldwide interconnection of autonomous systems that keeps the Internet running. Yes, you need an Autonomous System Number (ASN) to use BGP (Border Gateway Protocol). ASNs allow networks to identify themselves and exchange routing information with other networks. An Autonomous System Number (ASN) is a unique identifier assigned to an internet network (Autonomous System) to manage and exchange routing information with other networks.