I'll bite. You probably (confirm by reading your router's manual) have to put your internal IP into that field (in this case 10.1.1.2) and then save the settings (and perhaps restart the router).
Then you can be the IPXNET server (IPXNET STARTSERVER <port>). Others can connect to your Dosbox with your external IP (IPXNET CONNECT <address> <port> ). Others don't have to change their router settings. Make sure your (and others') software firewalls (e.g. Windows firewall) let Dosbox through.
DMZ makes your computer visible to the Internet. Instead of DMZ, you can use those Virtual Servers to redirect only certain UDP ports (i.e. the one that you set your IPXNET server to listen to) to your computer.
PS: it's easy to disable your internet connection with those settings.
PS2: this is assuming that other players are across the Internet. If they're on your LAN (or otherwise inside your network address translated domain), they have to use your internal IP to connect.