Printer address changing

We have 4 computers(2 XP and 2 Windows 7) hooked up to a network and one printer. The problem we are having is that the printers Ip address keeps changing on three of the computers (One XP and 2 Windows 7) The one XP is the main computer that the router is hooked up to. The router keeps the correct address its just the computers that keeps changing it. On this computer I click Devices and Printers, Then choose the printer click open then customize your printer. The clcik the ports tab. The port is still set to the one we started off with. Then I click Configure ports. The were it says Printer Name or IP Address. This is where it keeps changing. Can't understand why it only changes on three computers and not all 4. What could be causing this and how do I fix it?