Internet Interconnection and Exchange
As mentioned earlier, NSPs can interconnect at one of several points. First, four NAPs in the United States are partially funded by the National Science Foundation (Ameritech, PacBell, Sprint, and MFS Datanet), and the reason they get some support from the U.S. government is that they also provide interconnection into Internet 2, the academic research backbone. Second, connection can be made via agreements between ISPs (that is, peering). Third, there are commercial interconnection and exchange points, which can be used through private NAPs.
Remember that the local service providers typically connect into the NAPs, or exchange points, through top-tier ISPs rather than directly. So what's the definition ...