An Autonomous System Number (ASN) uniquely identifies every network on the Internet. An Autonomous System (AS) is a group of IP networks operated by one or more network operators, with a single and clearly defined external routing policy. The ASN both identifies the network and enables it to exchange routing information with other ASes.
Every Internet service provider (ISP) requires their own ASN, but also the individual organizations that connect to the Internet through an ISP require one. After an application from an organization, ISP or other entity has been approved, ASNs are assigned by regional Internet registries. Any network that wants to peer at a public Internet Exchange must have a public ASN.
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