Gateway is an router that knows about other regions. A node in each region is designated as an entry point , and the entry point knows how to reach the entry points in all the other regions. Sub optimal routes. The average path length increases because there may be a shorter path that bypasses the entry points, but we don't use it. Hierarchical routing can be extended to multilevels.
Area code identifies a region.
Area code plus the first three digits identify the central office within a specific region. Broadcasting Sending a packet to all destinations simultaneously is known as broadcasting . Send a unicast packet to each destination. However, this approach makes poor use of resources. Flood packets to all nodes.