Press the windows key on your keyboard, or click on the windows(start) button.
Once this is open type the following "cmd" this will find the Command Prompt application. If Command Prompt does not show up in the list of applications you can find it in All Programmes -> Accessories -> Command Prompt.
When it has opened type in the following "ipconfig" followed by enter.
This will then print a bunch of text. The part that your interested in is the line with the words IPv4 Address the numbers circled below is where your subnet will be listed.
Go into System Preferences -> Network
Depending on the network connection type you can find your IP address for a Wi-Fi connection here
And for a Ethernet connection here
The subnet is the first 3 ranges(###.###.###)of your ip address So for the examples above the subnet will be 192.168.1
Open the terminal and type "ifconfig" and hit enter.