BGP is literally the routing backbone of the Internet. As we all know it, the Internet consists of millions of interconnected networks. In the telecom industry, these millions of individual networks are referred to as Autonomous Systems (ASs). Each AS is managed under a single administrative domain (e.g., one organization or an ISP), with its own unique AS number and IP address pools aka IP prefixes. The AS number can be private (i.e., not visible publicly), and so can be the IP address pools. For example, when multiple branch offices of one company interconnect, they can use a private AS number and IP prefix for each branch office. Networks that want to use a public AS number and publicly routable IP addresses have to apply for them at a Regional Internet Registry (RIR) like ARIN, APNIC, RIPE. The RIR assigns a unique AS number and IP prefix(es) to that network.