Select a D-Link DL-3100 bridge/router (RAD Network Devices OEM) 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 locate rndImageSize under the rndDeviceParams branch of DLINK-3100-DEVICEPARAMS-MIB, then select it and click OK. It reports the number of flash sectors currently allocated for firmware image storage. 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 INTEGER-type OID, Value bounds is the most useful condition here — trigger an alert if the value approaches the flash device's total sector capacity, so you catch a full or nearly-full image partition before a firmware upgrade fails. Click Finish to create the monitor; you can adjust any parameter later.