Using the TCP/IP configuration GUI you are limited to 6 IP addresses however more can be added by directly editing the registry:

1. Log on as an Administrator
2. Start the registry editor (regedt32.exe)
3. Move to HKEY_LOCAL_MACHINE\SYSTEM\
CurrentControlSet\Services and scroll down to the service for your adapter card (Look at the adapters tab on the Network Control panel applet). For example the Etherlink 3 card is Elnk3, however you want the first occurrence so goto Elnk31.
4. Move to the Parameters\TCPIP subkey
5. Double click the IPAddress value. Enter in additional IP addresses separated by a new line
6. When finished click OK
7. Next edit the SubnetMask and again add an entry for each IP address added (in the same order). Click OK when finished.
8. Close the registry editor
9. Reboot the machine