In the web interface of the DD-WRT firmware (v24-sp2 mega - build 14896) under Setup -> Networking -> Port Setup there are fields for an IP address and a subnet mask. They appear at the bottom of that section, after the options for multicast forwarding and Masquerade / NAT. The following is a link to a screenshot showing the fields.
The DD-WRT help page for this page does not state anything about these fields. What is their purpose and which IP address and subnet mask should be entered there in most cases (i.e., in a standard configuration)?