Cacti and other Network monitoring tools requires SNMP (Simple Network Management Protocol) to monitor your server's network bandwidth, cpu loads, total ammounts of RAM and Hard Drive etc. To monitor your Linux server, you must have net-snmp installed.
Installing Net-SNMP on Debian or Ubuntu
For Debian, Ubuntu and other Debian-based distro, type this command:
apt-get install snmpd
Edit the /etc/default/snmpd file
vi /etc/default/snmpd
find this line