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We review what we've covered so far for the OSI reference model in this section and then introduce another, more condensed networking model, known as the TCP/IP model. The important point to remember about the 7-layer OSI model is that its primary purpose is to serve as a set of interoperability standards for vendors of networking hardware and software. This is based on well-defined responsibilities at each layer and allows for seamlessly replacing one brand of equipment for another. We then have a look at the TCP/IP reference model. Instead of 7 layers, it condenses the same functionality as the OSI model into just 4. It's a model that had its beginnings in the DoD. The mapping of OSI layers to those in the TCP/IP model are then presented along with a handy reference sheet. Again, as with the OSI model, important concepts to focus on for the CISSP exam are the functions (protocols) that exist at each layer and also the attacks that go along with them. In conclusion, the important point to remember is that you must know the OSI reference model inside and out! This is important since if a particular reference model is not clearly stated on the exam, then assume the default is the OSI model.