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Cacti (cacti.net) is a network monitoring tool that graphs system and network information over time (time-series data) and provides a comprehensive Web interface for browsing and examining the ongoing performance of the devices on a network.
For example, you can configure Cacti to monitor the network traffic passing through the network ports on local servers and the switch and router ports on the local network. Cacti graphs provide information on traffic levels on the various parts of the network. When the network is slow, you can refer to the historical graphs and see if anything out of the ordinary has occurred. In addition to network traffic levels, Cacti can collect data on CPU utilization, disk space usage, page views on a Web server, and almost any other data points available on the local network.