Context: ISPs may engage in peering, where multiple ISPs interconnect at peering points or Internet exchange points (IXs), allowing routing of data between each network, without charging one another for the data transmitted—data that would otherwise have passed through a third upstream ISP, incurring charges from the upstream ISP.

Question: What is peering? 
Answer: multiple ISPs interconnect at peering points or Internet exchange points

Question: What does peering allow? 
Answer: routing of data between each network, without charging one another for the data transmitted

Question: Why is peering used? 
Answer: data that would otherwise have passed through a third upstream ISP, incurring charges from the upstream ISP

Question: What are IXs? 
Answer: Internet exchange points

Question: Where do multiple ISPs connect? 
Answer: peering points or Internet exchange points

Question: Who may not engage in peering?
Answer: Unanswerable

Question: Different networks charge each other for what?
Answer: Unanswerable

Question: What would've been incurred by data passing through a third upstream ISP?
Answer: Unanswerable

Question: Nothing to what costs ISPs more money?
Answer: Unanswerable