A postal address has three components that can be used to deliver mail: state, city, and street. A phone number has an area code and exchange. At the core layer, mail can be delivered to the next post office based on only the state or city and state information. A phone number is delivered at the core layer based on the area code.
A default route is used if there is not a specific entry in the routing table for the destination.
Routing moves a letter or telephone call to the access layer (as in a street or telephone exchange). Switching makes the final delivery. A switching decision is made on the part of the address that is not used in routing (as in the street number or last four digits of a phone number).
At the core layer in the postal system, the only information that is needed to make a routing decision is the state or city/state information. The specific street names and street numbers are hidden, the core layer does not need this information. At the core layer in the telephone system, the area code is used to make a routing decision. The specific exchange or last four digits of the phone number are not needed, or hidden, from the core layer.
If a delivery system is not divided into access, distribution, and core layers, every point in the system needs to maintain every possible destination address to make a delivery decision. The use of a layered system means each layer needs only the information necessary to deliver to the next layer, either above or below.