BGP and routing on the internet

In short: The Border Gateway Protocol (BGP) exchanges between the large networks of the internet (autonomous systems) which IP ranges they can reach. That is how packets find their way around the world.

In more detail: Each autonomous system (AS, for example a provider) has a number. Via BGP it announces prefixes (“I reach 203.0.113.0/24”) and passes on learned routes. Routers choose the best path by rules and policies. BGP has no built-in security and trusts announcements.

In Depth

• AS path: List of traversed AS. Shorter is usually better, policies (price, contracts) override.
• Peering and transit: Equal exchange vs. paid carriage.
• Route hijacking/leaks: False announcements redirect traffic or take services down (examples: YouTube 2008, Facebook outage 2021 after routes were withdrawn).
• RPKI: Cryptographically proves which AS may announce a prefix.
• Anycast: Same address in many places; BGP leads to the nearest.
• IGP vs. EGP: OSPF/IS-IS inside, BGP between organisations.

See also: Anycast and load distribution on the network, Subnet