I see here that even some organisation/universities have bought ASN which are located at single location. What benefit they will get by buying ASN ? Is it compulsory to buy ASN if I have to buy some public IP's from my ISP ?
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An Autonomous System number (ASN) is required to use the BGP protocol. An organization can get a registered ASN, or it can use a private ASN (similar in concept to private IP addresses).
ISPs use BGP to exchange routing information with other entities. So if your organization wishes to exchange routes with your ISP, you will need to run BGP, and you will need an ASN.
If the routes you advertise are registered to you (i.e., provider-independent space), you will need a registered ASN. If you are using address space provided by your ISP, you may be able to use a private ASN. That is something you would negotiate with your ISP.
An organization that has only a single connection to one ISP, in most cases, doesn't need to use BGP, and can simply use static routing.