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 and IXP.Tier 2 and Tier 3 NetworksTier 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obtain access to the larger backbones.ARPAnet to NSFnet to Tier 1ARPAnet was the original Internet backbone, which was superseded by the National Science Foundation Network (NSFnet) in the late 1980s.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, NSFnet and Internet.