Why a Layered Model? Why 7 Layers?

Why 7 layers, why not 3, why not 10?
The following principles were followed :

• A layer corresponds to a different level of abstraction.
• Each layer provides a well defined, independent function.
• Within each layer unique protocol standards should be enforceable.
• There should be a minimum of traffic between layers/across interfaces.
• The number of layers should be sufficiently large that distinct functions are in different layers and that there are not too many layers.

Today we could argue that networking involves two additional layers:

8. Politics
9. Money