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Part VI: Appendixes > OSI Model Overview - Pg. 661

APPENDIX C OSI Model, TCP/IP Architecture, and Numeric Conversion The Open Systems Interconnection (OSI) model is a mandatory topic in any internetwork- ing book. The CCDA candidate should understand the OSI model and identify which OSI layers host the different networking protocols. The OSI model provides a framework for understanding internetworking. This appendix provides an overview and general under- standing of the OSI reference model. The Transmission Control Protocol/Internet Protocol (TCP/IP) architecture provides the practical implementation of a layered model. This appendix provides an overview of the TCP/IP layers and how they map to the OSI model. Also covered in this appendix is the numeric conversion of binary, decimal, and hexadeci- mal numbers. The ability to covert between binary, decimal, and hexadecimal numbers helps you manipulate IP addresses in binary and dotted-decimal format. Quickly convert- ing these numbers will help you answer test questions. OSI Model Overview The International Organization for Standardization (ISO) developed the OSI model in 1984, and revisited it in 1994, to coordinate standards development for interconnected in- formation-processing systems. The model describes seven layers that start with the physi- cal connection and end with the application. As shown in Figure C-1, the seven layers are physical, data link, network, transport, session, presentation, and application. The OSI model divides the tasks involved in moving data into seven smaller, more manage- able layers. Each layer provides services to the layer above, performs at least the functions specified by the model, and expects the defined services from the layer below. The model does not define the precise nature of the interface between layers or the protocol used be- tween peers at the same layer in different instantiations of a protocol stack. The model's design encourages each layer to be implemented independently. For example, you can run an application over IP (Layer 3), Fast Ethernet (Layer 2), Frame Relay (Layer 2), or Gigabit Ethernet (Layer 2). As the packets route through the Internet, the Layer 2 media change in- dependently from the upper-layer protocols. The OSI model helps standardize discussion of the design and construction of networks for developers and hardware manufacturers. It also provides network engineers and analysts with a framework useful in understanding internetworking.