In this document you will be able to find theoretical and practicing information about how to use GNS3, a network simulator software. This document also contains some lines about the previous concepts that users must know before using this kind of applications, concepts like virtualization or IOS. You also can find some concepts or protocol descriptions in the document. Those descriptions are in the parts when new words or concepts appear. After the introduction part, you will find some steps informing about the different options that GNS3 have, how to configure them and how to work with them. In the whole document you can find some limitations that this network simulator have, every one of them is the specified part that are referred to, but obviously this document is focused on what GNS3 can do. At the end, you will see some practicing, simple examples about how to use the application. In the first exercise, a spanning tree protocol is configured. The objective is to check if GNS3 is able to adapt to topology changes after modifying the root switch or after shutting down an interface. The second practice is a VLAN scenario. There are two different VLANs configured and two methods to do it. You will be able to find both ways to configured and a connectivity test in each case. The third and last part is a backup connection configured with static routes. Once is configured, we'll make a ping to check if there is connection and once we have the answer, we'll disable the principal link while we're making a ping. We'll be able to see that the traffic stops going one way and starts using the backup link.