In January 1994 there were four public fiber-optic backbones in the U.S.: NSFNET, Alternet, PSInet, and SprintLink. The NSFNET was funded by the NSF; it evolved directly out of ARPANET, the original TCP/IP network. The other backbones were private, for-profit enterprises.
By summer 1995 there were at least 14 national and super-regional high-speed TPC/IP networks in the U.S. As interconnection proliferates, the distinction becomes less important. A map of the major interconnection points and the numerous networks that use them is available at CERFNET.
MCI, which helped operate the original NSFNET, is probably the largest carrier of Internet traffic today; they claim to carry 40% of all Internet traffic. However, this is a highly competitive market; Sprint, Alternet, and PSInet are also signing up many customers.