In the Admin UI under Server Network Settings, there's the option to set the specific interface that the OpenVPN daemons should listen on. These are the programs that handle any incoming OpenVPN tunnel connections. This can for example be set to all so that it will simply listen to all available network interfaces, and usually, this is the default. But in some situations, it is desirable to configure the OpenVPN daemons to listen on a specific network interface only. You can also change the ports that the OpenVPN daemons listen on here, but we normally recommend not changing these unless there are unique circumstances.

Advanced option settings on the command line

Access Server Resources:
OpenVPN Access Server Documentation
OpenVPN Access Server Resource Center
OpenVPN Access Server Admin Manual