Context: Extensive and rapid continuous growth in Internet Protocol (IP) based networks and as the result of increasing dependencies on these networks makes them extremely challenging to manage and keep them running all the time. 24/7 non-stop monitoring is important to minimize the down time of the network. For this reason dependency on automated network monitoring has been increased. Objectives: There are many tools and systems available for network monitoring. This includes expensive commercial solutions to open source products. Nagios and OpenNMS are two of the most popular systems and they are considered to be close competitors. Comparison discussions about them are very common at different forums on internet. But no empirical study for comparison analysis has been done. In this thesis the comparison study between Nagios and OpenNMS was conducted. Methods: Network monitoring functionalities are listed down from literature followed by industrial interviews with the networking professionals. Then taking these functionalities as a base, to evaluate, survey studies for both systems were conducted separately and comparison analysis of the results was performed. Usability evaluation of both systems was done by conducting usability testing and comparison analysis of the results was performed. Results: Besides providing the comparison of both systems this study also can help to findout the strengths and weaknesses of both systems separately. And in the end we suggested a list of functionalities and features which might help in selection of a monitoring system and also might be helpful for improvement of the monitoring systems.