Source : Free On-Line Dictionary of Computing

hierarchical routing
     
        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.