What do all the settings mean?
Choose at least one VPN type, PPTP or L2TP to determine which type of VPN server to run. L2TP is typically more secure so it is advised to use this. But, PPTP is sometimes more compatible with routers, especially when using Windows clients. You may find it useful to run both.
To authenticate your VPN clients, they must connect using a username and password. In iVPN, there are two methods of authentication; using a directory server or using custom accounts. Custom accounts are the easiest to set up as they do not have to correspond with your computer's user accounts; these are completely separate. All you have to do is add at least one username and password and you're done. You can also import and export custom user accounts with the choice of encrypting them for security. Using directory authentication is slightly more complicated. First you must have a directory server already set up such as Mac OS X Server's Directory Service. Once you have confirmed this, you must join that directory server from the Mac that iVPN is running on to add your directory server to the list of available directory servers. You can then use the "Manage Directories" button in iVPN to choose which directories to use for authentication. If you're not exactly sure how to use a directory server for authentication, I would strongly advise you use custom accounts.
You have to enter this if you chose to use L2TP IPSec. This secret is just a password that is used to encrypt your connections. Make sure you use something complex but memorable. E.g. Smith1email@example.comC. The shared secret can be stored in the keychain (recommended).
IP Address Range
This section allows you to designate a range of IP address for all of your clients. This can be any valid IP range ( e.g. 192.168.1.100 to 192.168.1.200 ). In this case, when the first client connects, they would get the first available IP address, which would be 192.168.1.100. When the next client connects they would get 192.168.1.101, etc.
Primary and Secondary DNS Servers
Unless you have specific DNS servers you would like to assign to your clients, leave these at their defaults (18.104.22.168, 22.214.171.124). If you have several DNS servers you would like to use, or you would like to provide custom search domains to your connected clients, you can do this in the Advanced DNS panel.