For your CCNA Exam ( ICND1 and ICND2 ) you are going to need to know (inside and out) the OSI model which is the fundamentals of networking.

Under its official name, the Open Systems Interconnection Reference Model, or the OSI model, was developed by the International Organization for Standardization, which uses the abbreviation of ISO in 1984. And, yes, the full acronym of the OSI is ISO OSI.

The OSI model is a layered model that describes how information moves from an application program running on one networked computer to an application program running on another networked computer. In essence, the OSI model prescribes the steps to be used to transfer data over a transmission medium from one networked device to another. The OSI model is a seven-layer model developed around five specific design principles:

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