Let, Know about Network Model.

Network Models

There are two theoretical models of networking, the TCP/IP Model and the OSI Model.
The TCP/IP Model directly reflects how the Internet and TCP/IP based networks work. As the smaller, less complex model it is easier to learn. However, engineers more frequently refer to the OSI Model.
The OSI Model is a model of how networks should work. It is theory only, and not a hard science. It is a model designed to make it easier to categorize different technologies and protocols so that their operation can be more easilly described and understood.
Both the TCP/IP Model and the OSI Model are part of the Cisco CCNA certification exam, so you have to know them both.