A top-level network on the Internet. There are about a dozen Tier 1 networks, most of which are in the U.S., including AT&T, Global Crossing, Level 3, Qwest, Sprint and Verizon (originally UUNET). Known as "settlement-free peering," Tier 1 networks are private networks that allow traffic from other Tier 1 networks to transit their backbones without a fee. See peering
Tier 2 and Tier 3 Networks
Tier 2 networks peer with some networks without fees but pay to reach a large portion of the Internet. Tier 3 networks always pay fees to obtain access to the larger backbones.
ARPAnet to NSFnet to Tier 1
ARPAnet was the original Internet backbone, which was superseded by the National Science Foundation Network (NSFnet) in the late 1980s. NSFnet was replaced by Tier 1 networks in the mid-1990s when the Internet became a commercial venture. See ARPAnet