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What is an Autonomous System Number (ASN)?

Rinus Reuvekamp

An Autonomous System (AS) is a collection of IP networks that are managed as a unit and connected via Internal Routing Protocols (IGPs). Each AS in this group is assigned a unique ASN. An autonomous system (AS) is an integral part of the life of internet providers.

It seems complicated, but let's simplify a little: In fact, the internet is a set of all devices (computers, tablets, smartphones, servers, etc.) that are interconnected directly or indirectly in a network. To identify and manage to exchange information, each device has an IP address.

How does an Autonomous System work?

The internet only works with this IP address and to organize this addressing, there is a worldwide organizing entity that is the Internet Assigned Numbers Authority (IANA). As in a city where to define the address of each house, we have the public administration that defines the rules and the addresses, the IANA makes this organization for all the Internet addresses (IP).

When becoming an AS, the provider has several advantages such as: use of portable IP addressing , greater amount of IP addressing and permission to allocate valid IP addresses directly to clients, which improves the use of some applications, facilitates the process of (security) and increases the possibility of implementing redundancy of access and traffic exchange agreements .

Types of Autonomous System Number

Generally, there exist two types of ASN, which are private and public. A public ASN is typically used when the system exchanges the routing data on the public internet in connection with other autonomous systems. On the other hand, a private ASN is used when your autonomous system communicates with a single service provider through BGP. 

So, when it comes to public ASN, it is important to know that the routes will become visible on the internet. In contrast, as far as private ASN is concerned, the internet will not reveal the routes. 

Significance of Autonomous System Numbers

An AS is a set of networks and routing devices managed by a single organization. The Autonomous Systems communicate with each other through routers, they exchange information to update their routing tables through the BGP or EGP protocol and exchange Internet traffic that goes from one network to the other. 

The ISP is an organization or provider of Internet services that are responsible for giving the autonomous systems IP addresses for the routing of the information of this. Even considering that the ISP could support multiple autonomous systems, the Internet only considers the trajectory definition policy established by the ISP. 

The autonomous systems are given a number that identifies it as unique, this is a 16-bit integer, which allowed a maximum number of 65536 autonomous system assignments. The Internet Assigned Numbers Authority (IANA) is in charge of assigning it, along with the ISP, the numbers that identify it, until now it had been assigned a range of private addresses from 64512 to 65534. 

The range of ASNs from 56320 to 64511 and 65535 are reserved by the IANA and should not be used in any routable environment. ASN 0 can be used for non-routable networks.