To open all the 16 port in IDM you need to do some simple step.
Step 1:
Open IDM by dobule clicking the IDM icon from task bar
OR
From Start Manu > All Program > Internet Download Manager > Internet Download Manager
Step 2:
Click on Option from the Graphical menu on the top
Or
From the menu bar Download > Option
Step 3:
A new window will be open with various tab on the above.
Now click on "Connection" Tab.
Step 4:
Hare you will find "connection type / Speed.
From the drop down menu select "LAN 10 MBs.
Step 5:
Just bellow the above counter, you will find "Default max. conn. number".
Here from the right side drop down menu select "16".
Step 6:
Click on "OK" at the bottom of the window.
That's all my friend, If your internet connection has appropriate speed then Your IDM will open all the 16 port.
Hope this will help you.
Thank you.