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The list can be viewed at menu -> Manage -> Networks -> List Networks.
Networks can be added to the list at menu -> Manage -> Networks -> Create Networks.
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You can disable the feature in the config (menu -> Admin -> Community -> All Configuration) by changing the value of blessed_subnets_use to 'n' (sans quotes).
New Installs
On a new install, the feature is enabled and the typical user should see nothing different. Running discovery will work as normal.
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You may need to populate the networks in advance if you are running remote audit hosts (other Open-AudIT instances) that submit back to the main instance. This is an unusual setup and Opmantek are only too happy to help with support if required.