Network connectivity is usually an issue on a local scale: one router or modem or device has a broken component---usually software that needs to be reset in one way or another. However, it rears its head on a very large scale as well: routing connections, as mentioned earlier, relies on the ability to communicate with at least one ``neighbor'' on the network, which is fine unless that neighbor goes down for whatever reason. And in non-decentralized systems, such as modern ISPs, that's exactly what happens: a software bug or power outage or any sort of problem tanks an entire area's coverage for hours to days.

\sinclude Combinatorics Applications:net-apps