The US portion of the Internet is best thought of as having three levels. At the bottom are local area networks (LANs); for example, campus networks. Usually the local networks are connected to a regional, or mid-level network. The mid-levels connect to one or more backbones. A backbone is an overarching network to which multiple regional networks connect, and which generally does not serve directly any local networks or end-users. The U.S. backbones connect to other backbone networks around the world. There are, however, numerous exceptions to this structure.
A few years ago the primary backbone was the NSFNET. On April 30, 1995 the NSFNET ceased operation and now traffic in the US is carried on several privately operated backbone. The new ``privatized Internet'' in the US is becoming less hierarchical and more interconnected. The separation between the backbone and regional network layers of the current structure are blurring, as more regionals are connected directly to each other through network access points (NAPs), and traffic passes through a chain of regionals without any backbone transport.