The first thing you are strongly recommended to do for a server is to give it a static IP address. This IP is only used with the router, and is not related to the global IP address actually used to access the server from afar.
Without a static IP, you would be forced to change the router port forward settings every single time the computer was disconnected, so it is very useful.
In Debian, edit
/etc/network/interfaces and add the following:
Note: Make sure to change
addressto the IP address that you want (since this is a router subnet, it must be
192.168.1.x, and must not be in use).
Make sure that
gatewaypoints to the router's IP address, which is usually
192.168.1.1, but may differ.
# static IP settings auto eth0 allow-hotplug eth0 iface eth0 inet static address 192.168.1.11 netmask 255.255.255.0 gateway 192.168.1.1
Now restart the server, and the server should now be using the static IP in question (unless there was a conflict, if the address was already in use).