Hello everybody. I have managed to install and configure properly SNMP. I have read this forum and followed the instructions provided by this site. Everything was fine but yesterday I tried to install and configure SNMP on another machine (Red Hat) and met these problems:
1) The folder /etc/snmp/ was missing so I thought to create it and write the snmpd.conf file.
2) Later I found out that the "/etc/init.d/snmpd start" command didnt work because snmpd was missing.
I typed "service --status-all | grep snmp" to see if the SNMP service is running and I got this error:

FATAL: Could not determine fully qualified hostname. Please set 'visible_hostname'

Squid Cache (Version 2.5.STABLE1): Terminated abnormally.
CPU Usage: 0.010 seconds = 0.010 user + 0.000 sys
Maximum Resident Size: 0 KB
Page faults with physical i/o: 385
/etc/init.d/squid: line 162: 658 Aborted $SQUID -k check

When I typed "ps -A" i could see that snmpd had 18970 as PID.
Any advice or suggestion would be really helpful. Thank you very much