Packet Tracer is a networking learning tool that supports a wide range of physical and logical simulations. It also provides visualization tools to help you understand the internal workings of a network. These tools will help you develop an understanding of how data flows in a network. Packet Tracer allows users to create large or small simulated network topologies by dragging and dropping routers, switches and various other types of network Appliances.
You can also use Packet Tracer to create your own experiments and networking scenarios. We hope that, over time, you consider using Packet Tracer – not only for experiencing the pre-built activities, but also to become an author, explorer, and experimenter.
Packet tracer software helps to simulate the network configuration of CISCO router, switches and many network security device for example ASA(Adaptive Security Appliance ) using Command line interface (CLI).
CLI is an interactive shell where you can write your command to configure your Router , Switches and other CISCO Compliances.
Packet racer can be installed in Linux, Windows even android and iOS apps is also available. Previously only the students enrolled in a CCNA Academy program could freely download and use Packet tracer software free of charge for educational use. But since August 2018 with version 7.2 it is free to everyone.
Here is the Download links for Packet tracer for LINUX and Windows version.