How To Install GNS3 (Graphical Network Simulator) On Ubuntu

1)
Install gns3
#sudo apt-get update
#sudo apt-get install gns3

2)
Download IOS from following link
http://kuba.stelmaszczyk.com/tmp/dynamips/IOS/

3)
Open gns3 and upload IOS file
Edit --> "IOS images and hypervisors" --> Settings --> Image files --> Select teh downloaded IOS file *.bin --> Save

4)
Test
a)
Goto gns3 GUI and drag the router from left nav for which you uploaded the IOS file into right side.

b)
Then right click on the router and select "Configure".
Configure the Adapters/Interfaces required for the router.

c)
Then right click on the router and select "Start"

d)
Then right click on the router and select "Console". This will open a Console where you can type the commands.