OSI Network Model

OSI NETWORK MODEL (7 layer model) :

 The communication between two hosts has been sub-divided into 7 layers by OSI Model.

Layer 7 - Application Layer    --  How the application decides whether it needs to communicate another host..

Layer 6 - Presentation Layer  --  How the data should be presented to the user for their understanding.

Layer 5 - Session Layer           --  How the application session is established or maintained.

Layer 4 - Transport Layer       --  How the communication must be established and maintain error free communication.

Layer 3 - Network Layer         --  How to reach the destination network, if both the hosts belong to different networks.

Layer 2 - Data-Link Layer      --  How to reach the destination host within the network.

Layer 1 - Physical Layer         --  How to send the data over the physical medium.

For ease of understanding we can divide these 7 layers into 2 sub-categories. such as 

Layer 7, Layer 6 and Layer 5 as Non-Communication Layers run on the host system. These layers are not concerned with the network communication

Layer 4, Layer 3, Layer2 and Layer 1 as Communication Layers. These are the basis on which the communication is established between two hosts. 

At Cisco, we are concerned only with the communication part of the OSI Model. We are not concerned how the applications run, present the data or the application's session is maintained (or how long it is maintained). We are concerned with the establishment of communication when an application wants to reach another application on a different host for data sharing or requirement, finding and resolving of the destination network, destination host etc. How is this achieved ? All the different aspects related to this end is our concern. So, now we know what is our duty.

All networks refer to OSI Model for learning or understanding the aspects of Network. Do they implement OSI Model in reality. To learn the answer to this we need to see the other model DoD Network Model.