Top free open source tools for network admins: PHP WeathermapSometimes you want the 10,000-foot overview of your network traffic. PHP Weathermap provides exactly that. It shows a logical map of your routers and the links between them, using different colors to indicate how busy each link is. PHP Weathermap is a good complement to an application such as Cacti or OpenNMS.
PHP Weathermap does not handle its own data collection, so you'll need to pair it with another application such as Cacti, OpenNMS, MRTG, or RRDtool. PHP Weathermap has a plug-in that helps it integrate into Cacti, with some options available in the Cacti preference panels.
You will need to edit the map config files to adjust the appearance of your maps. You can do this by hand, though PHP Weathermap offers a GUI editor that runs within a Web browser. You can use the editor to create your nodes (routers and switches) and links, as well as produce a functioning map.
When you're ready for manual tweaking, you can add custom background images to the map and insert custom icons for your routers. You can also add subnodes, which allow you to display more information within the router's icon, such as CPU or memory usage. You can also insert parallel links or bonded links between routers.
Through further tweaking of config files, you can fine-tune the placement of router icons and the map's legend. If you have a more complicated map with plenty of router icons, you can create curved link lines between your router icons to help keep the map readable and less cluttered or cramped.
Top free open source tools for network admins: NtopNeed an sFlow or NetFlow collector to get a thorough look at your data traffic flows? Ntop will take NetFlow or sFlow data from popular switches and routers and display it in a Web GUI, complete with clickable links that take you to details about particular hosts or protocols or to actual conversations and flows.
There are interesting features to Ntop's output, such as identifying workstation users by their email addresses and (passively) detecting the operating systems of network hosts through packet analysis. Ntop will break down traffic volume on a per-protocol basis, helpful for comparing the traffic your network actually has to what you think it should have.