If you are interested in computers and hardware or you like to experiment with connections between routers or switches, then you are bound to appreciate the feature set of GNS3, as it enables you to set up topologies of routers (like IOS and JunOS), firewalls (such as (ASA or PIX) and hosts without risking to break down your equipment.
GNS3 supports several types of routers and switches, so it can help you study for certification exams such as Cisco CCNA, CCNP, CCIP, CCSP, CCVP and CCIE, Juniper JNCIA, JNCIS or JNCIE, while being equally useful to network engineers or admins.
Once you install the application, you can start creating your own project from scratch and add the objects you need by dragging and dropping them from the devices panel onto the dedicated area.
Next, you can establish relations between them as you see fit - when you try to create a faulty connection, GNS3 displays a warning message so you can correct your mistake. Due to this, you can rest assured that the topology you come up with can be applied on real devices without any glitches.
You can adjust the attributes of each of your objects to helps them match your exact requirements, and test the configuration every step of the way by running a simulation. You can even customize the display within the main window of GNS3 by hiding layers, interface labels or hostnames.
In a nutshell, if you are studying for a Cisco or Juniper certification exam, or you want to test a certain topology you created, you will surely appreciate the functions of GNS3. And since it is a freeware utility, you can just go ahead and discover its features yourself.