The complex problem of routing on large networks can be simplified by breaking a network into a hierarchy of smaller networks, whereeach 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.
