Select a Unix/Linux server running the Net-SNMP (net-snmp/snmpd) agent as the target host to create a monitor — the SNMP service should be up and running on it. Click New Monitor, then check SNMP Custom on the Favorites tab, click Next, and confirm the host. On the next page, click Select... to open the built-in SNMP MIB Browser and type memTotalFree into the Find box to locate it in the OID tree, then select it and click OK. It reports the total amount of memory currently free or available for use on this host, in KB (covering both real memory and swap/virtual memory). On the monitor's Main parameters page you can set the target's SNMP port (default 161), credentials, polling interval, and other settings — see the SNMP Monitor help for details. On the State conditions and Alerting tabs, configure when the monitor should change state and trigger an alert; since this is an Integer32-type OID, Value bounds is the most useful condition here — trigger an alert if free memory drops below the level you'd expect for healthy headroom on this host, since persistently low free memory can precede swapping or OOM issues. Click Finish to create the monitor; you can adjust any parameter later.