The OSI reference model divides the problem of moving information between computers over a network medium into seven smaller and more manageable problems. Each of the seven smaller problems was chosen because it was reasonably self-contained and therefore more easily solved without excessive reliance on external information. Each of the seven problem areas is solved by a layer of the model.
Most network devices implement all seven layers.
To streamline operations, however, some network implementations skip one or more layers. As the information to be sent descends through the layers of a given system, it looks less and less like human language and more and more like the ones and zeros that a computer understands. This is necessary because each layer in a system has certain tasks it must perform. To perform these tasks, it must communicate with its peer layer in the other system. The OSI model's layering precludes direct communication between peer layers in different systems.