Most Transmission Control Protocol/Internet Protocol (TCP/IP) routing is based on a two-level hierarchical routing in which an IP address is divided into a network portion and a host portion.
Most corporate intranets consist of a high speed backbone network.
The reason this is a good arrangement is because even though there might be dozens of different workgroups, the span (maximum hop count to get from one host to any other host on the network) is 2.
Even if the workgroups divided their LAN network into smaller partitions, the span could only increase to 4 in this particular example.
Unrecognized destinations are passed to the default route.