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.