How the Network Field Is Used

Typically, only the network portion of the address is stored in the table. Using the hierarchical strength of the addressing keeps the routing table small and the lookup short. The routing process makes a decision based on the longest match. This ensures that if VLSM has been deployed, the most specific network is chosen. Cisco IOS code mandates that the longest match can be a /32 or 255.255.255.255 mask. This is a match based on the full host address and is used in specific situations such as an OSPF environment. It is not encouraged as a common configuration because the size of the routing table grows rapidly.

The routes in the table are held in an order that speeds up the lookup process, ensuring that the routing decision is streamlined.

Later in the chapter, in the section "How the Routing Table Is Kept Current and Correct," you will see how the networks are placed in the table and how path selection to a remote network is chosen.

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