Unable to connect to Job Server behind a firewall from DI Designer

I have a DI Job server on a test server that sits behind a firewall and I am not able to connect to from my desktop DI designer. Which option should be used for allocating the Designer Communication ports. Should I select " Allow designer to set the port for Job server communication", if this option is selected what ports will be used. Or if i specify a range what values should be used in the from: and to: