I guess you could add 0.0.0.0/0 but that would be everything, people will try and access your server so it is better to restrict the range.
If you have a static IP get your IP address by typing "whats my ip" into google, then you can enter it directly into the allow rule box.
If your IP is dynamic and changes, you might want to restrict it to your ISP by using the first two digits x.x.0.0/16 (i.e. if your IP is 188.8.131.52 you would put 184.108.40.206/16) perhaps your ISP could give you the address ranges it uses?
I can see you are using cPanel, I have found ConfigServer (CSF plugin at configserver.com) and cPhulk (built in to cPanel) useful to blacklist and prevent brutefotce attempts.