Here is a fantastic, though somewhat outdated overview [1]. Section 5 is most relevant to your question. The network topology today is a little different. Th

Here is a fantastic, though somewhat outdated overview [1]. Section 5 is most relevant to your question. The network topology today is a little different. Think of Level3 as an NSP, which is now called a "Tier 1 network" [2]. The diagram should show links among the Tier 1 networks ("peering"), but does not. [1] https://web.stanford.edu/class/msande91si/www-spr04/readings/week1/InternetWhitepaper.htm https://web.stanford.edu/class/msande91si/www-spr04/readings... [2] https://en.wikipedia.org/wiki/Tier_1_network https://en.wikipedia.org/wiki/Tier_1_network