There is no single network called the internet, and no master map of it held anywhere. There are tens of thousands of separately run networks, each one an autonomous system with its own number, connected to each other by agreement. The Border Gateway Protocol is how those networks tell each other which addresses they can reach.
Each network advertises the address prefixes it can reach to its neighbours, and every advertisement carries the list of autonomous systems it has already crossed. A router that receives several paths to the same destination chooses one according to policy, where commercial relationships weigh as heavily as distance, and then passes its choice on. Internet routing is the sum of a very large number of local decisions, not a central plan.
Three things to remember
- The internet is a mesh of autonomous systems, each identified by an AS number.
- BGP advertises reachability for address prefixes, and the AS path records which networks an advertisement has crossed.
- Path selection is driven by policy, so the route chosen is not necessarily the shortest one.
Related explanations
- What Does 255.255.255.0 Actually Do? (It’s Been Hiding in Plain Sight for Years)
- What Happens When You Hit Enter? (It’s Always DNS)
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