The Modern Internet Backbone
Because of the enormous overlap between long-distance telephone networks and backbone networks, the largest long-distance voice carriers such as AT&T, Verizon, Sprint, and CenturyLink also own some of the largest Internet backbone networks. These backbone providers sell their services to Internet service providers (ISPs).
Tier 1 Internet Service Providers (ISPs)
The Internet consists of access links that move traffic to high-bandwidth routers that move traffic from its source over the best available path toward its destination. This core is made up of individual high-speed fiber-optic networks that peer with each other to create the internet backbone. The individual core networks are privately owned by Tier 1 ISPs, giant carriers whose networks are tied together. These providers include AT&T, CenturyLink, Cogent Communications, Deutsche Telekom, Global Telecom and Technology (GTT), NTT Communications, Sprint, Tata Communications, Telecom Italia Sparkle, Telia Carrier, and Verizon.
Today, these large corporations provide the routers and cable that make up the Internet backbone. These companies are upstream ISPs. By joining these long-haul networks together, Tier 1 ISPs create a single worldwide network that gives all of them access to the entire Internet routing table so they can efficiently deliver traffic to its destination through a hierarchy of progressively more local ISPs.
Internet Exchange Points (IXPs)
Internet exchange points (IXP) tie the backbone together. Backbone (Tier 1) ISPs connect their networks at peering points, neutrally owned locations with high-speed switches and routers that move traffic among the peers.
Tier 2 and Tier 3 ISPs
Below the Tier 1 ISPs are smaller Tier 2 and Tier 3 ISPs. Tier 3 providers provide businesses and consumers with access to the Internet. These providers have no access of their own to the Internet backbone, so on their own would not be able to connect their customers to all of the billions of Internet-attached computers. Buying access to Tier 1 providers is expensive. So often Tier 3 ISPs contract with Tier 2 (regional) ISPs that have their own networks that can deliver traffic to a limited geographic area but not to all Internet-attached devices. In order to do that, Tier 2 ISPs contract with Tier 1 ISPs for access to the global backbone, and in that way make the entire internet accessible to their customers.