The easiest way to find free IP addresses on your network is to create a Ping loop.
Open a command line session
Enter ipconfig on Windows or ifconfig on Linux or Mac OS
Look for the Default Gateway entry in the results. Write down the first three octets. In the following example, that Default Gateway address is 192.168.0.1, so the first three octets are 192.168.0
Enter the following text, replacing the example IP address octets with those of your own network:
for /l %i in (1,1,254) do @ping 192.168.0.%i -w 10 -n 1 | find "Reply"
The results of this loop are all the currently used IP addresses on your network. A free IP address would be one that is not in this list but within the same range. So, pick an address that would lie numerically between the gaps in the series of addresses that are in the Ping loop output.