ISO/ OSI MODEL 

           In networking, a reference model defined by the ISO that divides computer-tocomputer communications into seven connected layers. Such layers are known as a ''Protocol stack".
     Each successively higher layer builds on the functions of the layers below, as follows:
Application layer 7 (Upper most)
    The highest level of the OSI model. It defines the way that application programs interact with the network, including database management, electronic mail, and terminal emulation programs.
Presentation layer 6: The presentation layer defines the way that data is formatted, presented, converted and encoded.
Session layer 5: The session layer coordinates communications and maintains the session for as long as it is needed, performing security, logging, and administrative functions.
Transport layer 4: The transport layer defines protocols for message structure, and supervises the validity of the transmission by performing some error checking.
Network layer 3: The network layer defines protocols for data routing to ensure that the information arrives at the correct destination node.
Datalink layer 2: The datalink layer validates the integrity of the flow of data from one node to another by synchronizing blocks of data, and controlling the flow of data.
Physical layer 1 (Lowermost): The physical layer defines the mechanism for communicating with the transmission medium and interface hardware.