I can't remember the location of the file. Something like /var/lib/openvpn/ipp.txt. Have a look in /etc/clearos/clients.conf. It has one line pointing to the file. Normally OpenVPN builds this file by itself as new clients connect and it uses this to try to give the same IP to the same client. From memory it uses the username from the certificate and an IP. The IP's increment by 4 every time. If you want to guarantee fixed IP's (mine don't seem to move) you can set the immutable bit on the file (chattr +I filename) so the system can't update it. There is no harm doing that. I believe it is the recommended way. You'd need to remove the immutable bit every time you wanted to edit the file to add a new host.