What is an Autonomous System number?
An autonomous system is a very important and interesting topic in the networking world. The first question is what is an Autonomous System? It is defined as a collection of routers or networking devices under a common administrator. Its range is from 1 to 65535. There are two types of Autonomous systems, Public and Private. Let us understand this with an example. Let’s assume there is one company ABC. That company has three branches in Mumbai, Delhi, and Chennai. Out of these three branches, Mumbai and Delhi branches uses JIO as their service provider and the branch in Chennai uses Airtel as his service provider. Now Mumbai and Delhi branches come under one Autonomous system, say 65000, as they have one common administrator i.e. JIO, and use private AS number as they do not want to connect to the outside world. And Chennai Branch comes in different autonomous system, say 65001, as it is having different service provider. Now if anyone branch in Mumbai or Delhi wants to communicate with a branch in Chennai it has to go through the internet. Now every company wants to hide their AS number from the outside world due to which public AS comes into existence. Public AS number is AS of ISP which is uniquely distributed to all ISP. So now when Mumbai or Delhi branch wants to communicate with Chennai branch they will go to their ISP JIO with AS 65000 and from JIO to internet with public AS, say 20, and again from the internet to ISP of Airtel with different public AS, say 30, and from ISP to company branch with private AS 65001. Now suppose we have another company XYZ, JIO as a service provider, that will have a different Private AS number, say 65003, because private AS is allocated to one administrator or company. But Company XYZ will use the same public AS to communicate as they are allocated to ISP. The range of public AS is from 1–64512 and for private AS it is from 64513–65535. To communicate in the same AS we use Interior Gateway Protocols (IGPs) such as EIGRP, OSPF, RIP, etc., and to communicate in different AS we use Exterior Gateway Protocols (EGPs) such as BGP.