Finding the Subnet Mask on Windows

• 1). On your keyboard, press the "Windows" key and "R" to open the Run dialog. Type "cmd" and press "Enter".
  
  
• 2). The Command Prompt will open. Type "ipconfig" and press "Enter".
  
  
• 3). Scroll through the results and identify the connection you are using. If you are connected to a wireless network, look for "Wireless Network Connection"; if your network is wired, look for "Ethernet Adapter". Your computer's IP address and subnet mask will be listed beneath the name of your connection.
  
  

Finding the Subnet Mask on Mac OS X

• 1). Click the "Apple" icon in the top-left corner of the screen, and then click "System Preferences".
  
  
• 2). In the "System Preferences" window, click the "Network" icon.
  
  
• 3). In the left-hand panel of the "Network" window, select the connection you are using, such as "Ethernet" or "AirPort". The current subnet mask will be displayed in the right-hand panel next to "Subnet Mask".