BGP
also: Border Gateway Protocol
BGP (Border Gateway Protocol) is the routing protocol used to exchange network reachability information between autonomous systems on the internet.
BGP is a standardized exterior gateway protocol that allows routers to advertise which IP networks they can reach. It's the protocol that powers internet routing, enabling different organizations' networks (autonomous systems) to communicate and find optimal paths to each other.
Unlike interior gateway protocols (like OSPF) that work within a single organization, BGP operates between organizations. Each organization announces its IP address ranges and the routes it can provide to reach those ranges. Routers then use this information to build routing tables and forward traffic across the internet.
Example: Your ISP's border router runs BGP to announce your company's public IP addresses to the internet, while also receiving announcements from thousands of other networks about how to reach their destinations.