right click on your network connection, CHOOSE PROPERTIES, click "internet protocol (TCP/IP)", click properties.
change the selector from "Obtain an IP address automatically" to "use the followin IP address"
then you should be able to enter your ip address, when you press enter, the subnet should be put in for you and SHOULD be 255,255,255,0
network IP addresses are normally 192.168.0.1 for the main pc/server and 192.168.0.(2 to 255) for the clients. all with a subnet mask of 255,255,255,0.