D-Link DSL-2640B - Manual (Page 50)

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Port Mapping
Port mapping is a feature that allows you to open ports to allow certain Internet
applications on the WAN side to pass through the firewall and enter your LAN. To
use this feature, mapping groups should be created.
Click on the Add button as displayed below. If you need to remove an entry, then
click on the Remove button.
After clicking the Add button, the below configuration screen appears, allowing you
enter the groups and the interfaces they are associated with.