The Internet (formerly ARPAnet) grew from a defense and research
institution-based network to a mass of interconnected nodes that are spread all over the
U.S. Many different providers have networks that span the country (and,
for that matter, the globe) but in order for the system to function
effectively these various networks must be connected at some point.
Here is a map showing the general layout of networks in the U.S.
and the "peering points" (i.e. the points at which networks are
connected to each other). The layout of the network allows network traffic to travel between networks and bypass links that might be temporarily down or busy.
This map is by no means the complete topology of the Internet in the U.S., for that visit CAIDA's MapNet for an Interactive Map.