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 autonomouSSyStem) knowS how to route internally. See alSo Exterior Gateway Protocol, Interior Gateway Protocol, tranSit network.