The OSI Reference Model comes from the International Organization of Standardization (ISO) and provides us a standard set of terms that we can use to discuss computer networking concepts. The OSI Reference Model describes the functions of a network as layers, seven of them. The layers are stacked and numbered from 1 to 7, bottom to top, like the stories of a building.

Layer 7, the applications layer, is where all network applications run. These are the applications that you use to send information, data, across the network, for example, web browsers and servers and E-mail clients and servers.

Layer 6, the presentation layer, controls the formatting of data for your application’s use. For example, if the data is sound from a radio broadcast to which you are listening across the Internet, the presentation layer formats the data for the application that you’re using to receive the broadcast.

Layer 5, the session layer, handles the establishment and termination of communication sessions. For example, when you login to your company’s mainframe, a session must be established for you to be able to talk to the mainframe, and, when you logout, your session is terminated.

Layer 4, the transport layer, provides source-to-destination connection establishment for data transfer. Over the connection, the transport layer can provide flow control and error checking.

Layer 3, the network layer, is the most important layer in routing. The network layer contains the address that routing software examines to determine where to route a message across a network.

Layer 2, the data link layer, is dependent on the medium to which a host is attached. The data link layer links the data from the network protocol to the physical medium on which it is to be transmitted.

Layer 1, the physical layer, defines the characteristics that belong to the type of physical network connection on a host, for example, the cable type, the connector type, the signal frequency, the signal level, and the maximum cable length. The physical layer determines exactly how bits (binary ones and zeroes) are transmitted and received on a network.

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