{$a}
The trusted hosts mechanism allows specific machines to execute calls via XML-RPC to any part of the Moodle API. This is available for scripts to control Moodle behaviour and can be a very dangerous option to enable. If in doubt, keep it off.
This is not needed for any standard MNet feature! Turn it on only if you know what you are doing.
To enable it, enter a list of IP addresses or networks, one on each line. Some examples: