Static IP
The internal network IP is not relevant to the ADSL router connection at all. It will be assigned an IP by the service provider automatically for the connection.
To set up a simple peer-to-peer network, proceed as follows:
What is important is that the router must have a static IP on the LAN. That will be something like 192.168.0.xxx, which must be unique on the network. All the other computers will have that IP set as the default gateway IP in the specific computer's LAN connection's TCP/IP settings. Your server and all the other workstations must have a unique IP in the same range, i.e. 192.168.0.xxx, where the xxx can be anything from 1 to 255. There are certain IP ranges which are reserved for LAN, such as 192.168.0.x, 10.0.0.x etc. Use one of those for the LAN only