To find the subnet mask number, access the Command Prompt via the Run dialog box, and run the ipconfig command that displays the data regarding your network connection. Alternatively, access the Properties dialog box of the network adapter via the Network and Sharing center.
Open the Run dialog box by pressing R while holding down the Windows key, type “cmd” in the empty field, and click OK. Once the Command Prompt window appears, type “ipconfig /all” next to the line that displays your username, and press Enter. Navigate to the Local Area Connection section, and look for a series of four numbers separated by periods next to the Subnet Mask entry. If this fails, run the Command Prompt as the administrator by typing “cmd” in the search field of the Start menu, right-clicking on the Command Prompt icon and selecting the Run as Administrator option.
To find the number without using the Command Prompt, open the Control Panel via the Start menu, click on the Network and Internet link, and click on Network and Sharing Center. Click on the Change Adapter Settings link in the navigation menu, right-click on the active network connection, and select the Status option from the context menu. Click on the Details button, and look for the number listed next to the IPv4 Subnet Mask line.