Russ White writing of Packet Pushers comments:
This bit of history should resolve one question network engineers ask all the time: why doesn’t the seven layer model match the TCP/IP suite of protocols? The simple answer is: TCP/IP was designed around a four layer model (the DoD model), rather than the world famous seven layer model. The seven layer model was not designed to be the end-all model for network operations. Seven was not a magical number. The model was designed to allow adding more layers as needed, over time. Why haven’t we added more? More on that in a moment.
Russ brings up an interesting point with TCP/IP model not matching the OSI model directly, but is that the fault of a conceptual model? OSI teaches more of the reasoning behind making multiple layers and what they do. Collapsing the traditional model for the sake of making it look like TCP/IP is going to cause more harm than good.
Read more at: The Seven Layer Model is Dead