Net-SNMP
The Net-SNMP application suite implements SNMP v1, SNMP v2c, and SNMP v3 using IPv4 and IPv6. It has a command line application to retrieve information from SNMP-enabled network-attached devices, and also manipulate configuration, and convert between numerical and textual MIB OIDs. It has a graphical MIB browser, a daemon application that receives SNMP trap notification, and a set of libraries to develop new SNMP application using C and Perl APIs.
Cacti uses Net-SNMP application suite to retrieve raw data from SNMP-enabled devices, and then uses the RRDTool to create graphics using raw data. In order to run Cacti the managing system needs to run Net-SNMP. If you need to install Net-SNMP manually, please see the Net-SNMP manual page: http://www.net-snmp.org/docs/man/.
These days, we don't need to install Net-SNMP manually in the managing system. There are distribution specific tools to install the application, such as yum in Fedora core, apt-get in Ubuntu. After installation, run...