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The complex problem of routing on large networks can be simplified by breaking a network into a hierarchy of smaller networks, where each level is responsible for its own routing. The Internet has, basically, three levels: the backbones, the mid-levels, and the stub networks. The backbones know how to route between the mid-levels, the mid-levels know how to route between the sites, and each site (being an autonomous system) knows how to route internally. See also Exterior Gateway Protocol, Interior Gateway Protocol, transit network.