Using multiple protocols is modular and allows changes to one protocol without affecting the others. For example, if the addressing protocol is dependent on the delivery protocol, changes to one would imply changes need to be made to the other.
The airport system. At the core routing level, there are major hub airports such as Denver, Chicago, New York, and Atlanta. The core airports are responsible for routing people and cargo to major geographical areas. Core airports connect with regional airports that serve a specific area; regional airports are at the distribution layer. Finally, to reach your final destination, you can take a bus, a cab, a train, or rent a car. This can be considered the access layer.
The source address is not used unless the letter needs to be returned to the sender. Using the destination address, the access level post office in New York examines the state, city, and street information to determine if it is directly connected to the destination. If not, the letter is sent to the distribution layer post office using a default route. The distribution layer post office also examines the state, city, and street information to determine if it is directly connected to an access layer post office servicing the particular street. If it isn't, the letter is routed to the core level using a default route. The core level post office examines the state name, and if the state name does not equal New York, the letter is delivered to the core post office for the state of California. The California core post office delivers the letter to the distribution post office that handles the city of San Diego. The San Diego distribution post office delivers the letter to the access post office that handles the destination street. Finally, the access level switch delivers the letter to the proper destination.
The street name and number are the access layer components. The city name is the distribution layer component. The state name is the core layer component.
The last four digits are the used at the access layer to identify a particular telephone. The next three numbers are used at the distribution layer to identify an exchange that services several phone numbers. The area code is used at the core level for routing between different regions.