In this paper theoretical basics of visualization are explained, necessary for quality visual object creation and its integration in the teaching process, which is generally a great challenge for the teacher. This paper brings previous works of authors who tried to visualize teaching materials in Informatics. The goal of this paper is to investigate using visualization in teaching Computer Networks. As there are too many abstractions in this area, theoretical concepts are basically too technical and difficult to master. This paper gives overview of many authors attempts to answer the question how to improve student's motivation and interest for the complex teaching material if Computer Networks. Visualization using the computer simulation is a simple, flexible and relaxed way to achieve the previous, but the investments are not trivial. From the teaching perspective, an important question arises: does the time and effort invested in visualizations give the useful teaching benefit? Analysis of data obtained in the experiment gives affirmative answer. Two simulators are chosen for the experiment, Packet Tracer and OPNET. Student positively reacted on using simulators in learning; both simulators helped them in better understanding of the theoretical concepts. However, based on the results on the post-test, greater improvement was achieved using Packet Tracer. Based on the results from the questionnaire and the student's comments, they are more satisfied with this tool than with OPNET.