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.