What is AS Number and Why do we need it ??

 The Autonomous system is basically a kind of TAG which is basically going to represent an organization, You as an administrator are the in-charge of the routing domain  like routing policies, Routing protocols etc etc .

An autonomous system number (ASN) is a unique number, assigned by IANA that is available globally to identify an autonomous system and which enables that system to exchange exterior routing information with other neighboring autonomous systems.

ASN Types
There are two types of autonomous system numbers - public and private.

Public ASN - Used when an AS is exchanging routing information with other Autonomous Systems on the public Internet. 

Private ASN - Used if an AS is only required to communicate via Border Gateway Protocol with a single provider. As the routing policy between the AS and the provider will not be visible on the Internet. [3] In this case the upstream provider will typically remove the ASN from the ASN Path and replace it with his own public ASN. In reality, this can be thought of as a type of NAT for ASN`s.

ASN Ranges
Below lists the various ASN Ranges:
0 : reserved.
1-64,495 : public AS numbers.
64,496 – 64,511 : reserved to use in documentation.
64,512 – 65,534 : private AS numbers.
65,535 : reserved.