What is an Autonomous System and an Autonomous System Number?
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The Internet exists because of Autonomous Systems (ASes), and each Autonomous System is identified by an Autonomous System Number (ASN). An autonomous system is a collection of connected Internet Protocol (IP) routing prefixes (or routers) under the control of one or more network operators on behalf of a single administrative entity, such as a university, government agency, or enterprise, that presents a common and clearly defined routing policy to the Internet.
Autonomous System Numbers are assigned to Local Internet Registries (LIRs) and end-user organizations by their respective Regional Internet Registries (RIRs), which in turn receive blocks of ASNs for reassignment from the Internet Assigned Numbers Authority (IANA).
The purpose of autonomous systems is to allow for the efficient routing of traffic on the Internet. By grouping networks together into ASes, network operators can control how traffic is routed within their ASes and to other ASes. This helps to ensure that traffic is routed in the most efficient way possible and that congestion is avoided.
The ASN is a 16-bit or 32-bit integer that is assigned to the AS by a Regional Internet Registry (RIR) or the Internet Assigned Numbers Authority (IANA). ASNs are used in Border Gateway Protocol (BGP) routing, which is the protocol used to exchange routing information between different Autonomous Systems on the Internet.
ASNs are important in network engineering and internet infrastructure management, as they help to identify and manage routing policies and interconnections between different Autonomous Systems. By exchanging routing information via BGP, Autonomous Systems can route traffic between each other and ensure efficient and reliable communication on the Internet.
Here are some examples of autonomous systems:
Google's AS is 15169.
Facebook's AS is 15189.
Microsoft's AS is 16384.
Amazon's AS is 16509.
AT&T's AS is 701.
Verizon's AS is 702.
Fun Fact: BGP is facilitated by the routers of these ASes. So imagine you have a Verizon FIOS and accessing Facebook. Your request when it reaches at the "Border" of the Verizon's AS, the last router runs BGP to find the shortest path to the AS of Facebook. So this shortest path can mean it will pass through the AS of XFinity, or Google or University of Maryland, etc. But after hopping from the border routers of these ASes, the request will reach facebook.
