Forum Privacy

I am setting up D7 for some an inner circle, therefore privacy is very important.  I tried to change items in all default_group to '4' which is under table privacy_actions.  I also want to restrict the Forum to be accessed only by login members.  Any hints I can do that?

also I found the sys_privacy_defaults table controlling the action by group_id. Can I make change on this table to do the forum by changing from '3' to '4'? However, I don't know what the action_id referring to? for example: I know 21 = video, but how can I found the others?