But even so there are some great best-of-breed freeware tools that I find to be quite good at helping with some...
aspects of network monitoring and management. It all depends on what you're hoping to accomplish with your management software. So I've decided to cover the basics in this long review of freeware or open source software. I did not include any shareware or partial, 30-day trials in my research. I will also state that the vast majority of network monitoring tools provide just enough basic functionality to entice you to buy their enterprise suite of products -- so again: caveat emptor.
For the best freeware network monitoring solution, I would recommend investigating PRTG, a modified hybrid of MRTG. It provides a comprehensive monitoring and mapping solution based on Simple Network Management Protocol (SNMP), NetFlow, Windows Management Instrumentation (WMI), and Switched port analyzer (SPAN) technologies. The biggest drawback is that the freeware version only supports 10 elements. That's hardly enterprise worthy but it does provide the full network monitoring capabilities for all 10 elements. Most of the other tools provided for more monitored elements but limited functionality or time-based trials. PRTG provides the ability to create and monitor based on sensors like HTTP, SNMP alerts, and NetFlow data into an integrated Web UI. It also has a great mapping tool that incorporates all monitored elements that I found easy to use and set up.
The vast majority of players understand SNMP and NetFlow, there is a lot more to monitoring a network for performance. One of the best free utilities I've personally used to diagnose performance problems is called QCheck. This utility is installed on two end points to do some comprehensive latency testing. I love this utility for its flexibility with various communication protocols and testing parameters. No network engineering toolset is complete without it. And it's free!
I would be remiss in mentioning that network performance often dictates that packet traces be conducted via span ports to diagnose critical problems. Most enterprise toolsets have some advanced functions to help pinpoint problem areas from a higher level but Wireshark is the staple for packet trace deconstruction for free. Wireshark incorporates deep inspection of thousands of protocols while allowing you to graph out TCP time sequence graphs. Did I mention it's also free? It's part of the GNU General Public License Agreement.
The final tip that I would recommend is to leverage your existing environment and infrastructure as much as possible to provide a comprehensive monitoring solution. Got Cisco? Did you know that IPSLA and NetFlow data is retrievable through the CLI? While the CLI doesn't have comprehensive graphical interface, baselines, thresholds, and alerting, the IP SLA data does support active transaction analysis for specific application protocols like HTTP and VoIP. So if your enterprise environment supports it, I would highly recommend leveraging as much of the management capabilities that are inherently offered.
For further research, take a look at ITPRC.COM's Network Management Freeware/Shareware applications and Stanford's frequently-updated list of network monitoring tools: two accurate lists of network monitoring applications and tools out on the market, ranging in price from free to extremely expensive.
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