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The list can be viewed at menu -> Manage -> Networks -> View List Networks.
Networks can be added to the list at menu -> Manage -> Networks -> Add NetworkCreate Networks.
Networks can have a description added by clicking the Edit Network link on the View Network page.
You can disable the feature in the config (menu -> Admin -> ConfigConfiguration -> All) 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.