The IP address of the master computer will keep changing (due to router power outages, for instance) if you don't configure the master workstation (server) to use a "static IP address". You have to reconfigure all of the client workstations to the new IP address whenever it changes. So we recommend configuring a static IP on the master computer (the other computers don't need a static IP).
Since this is a function of Windows, and of the details change slightly with every version, we can't provide a detailed walk-through. Ultimately we recommend having a computer professional set it up for you, but if you're willing to dig through some technical jargon then you can probably find the information yourself.. We've used Google to search for tutorials on the web, and came up with a few examples. We can't guarantee the links still work or whether they apply to your particular setup, so you might want to also try your router's documentation and/or your own Google search for "setting up a static IP in Windows".
IMPORTANT: Making mistakes in configuration can disable networking entirely, so if you're not the "experimenting" kind, then we suggest calling a computer technician to do it for you.