In networking, a subnet mask is an individually defined portion of a network. In your home, all of the computers on your network must have the same subnet mask to communicate with one another. If two computers have different subnet masks, they will be unable to share files, a printer or any other network services. Check the subnet masks on your computers as a troubleshooting step if your home network fails to function properly.
Finding the Subnet Mask in Windows 95 and Later
Press the Windows logo key (generally in the lower-left corner of the keyboard) and the "R" key simultaneously to bring up a new window labeled "Run." If you are using a keyboard without a Windows logo key, click the Start button in the lower-left corner of the screen, then click "Run" in Windows 95-XP. In Windows Vista and later, click the Start button, type the word "command" in the Search box at the bottom of the Start menu and click "Command Prompt" on the search results list.
Type "command" in the "Run" box in Windows 95-Millennium Edition or "cmd" in Windows XP or later. Press "Enter." The Windows command prompt will be displayed.
Type "ipconfig" and press "Enter."
Scroll through the results if you have more than one network adapter installed until you find the one that you are currently using (e.g. "Wireless LAN Adapter" or "Ethernet Adapter"), and examine the number shown next to "Subnet Mask" (e.g. "255.255.255.0"). This is the subnet mask used by your computer.
Finding the Subnet Mask in Mac OSX 10.5 and Later
Click the Apple logo in the upper-left corner of the screen, then click "System Preferences," or click the System Preferences icon on the dock if it is present.
Double click the "Network" icon.
Click the connection on the left side of the screen that you want to find the subnet mask of, e.g. "Ethernet" to check a wired connection or "AirPort" to check a wireless connection.
Click the "Advanced" button in the lower-right corner of the window.
Click the "TCP/IP" tab at the top of the window. The subnet mask is displayed under "Configure IPv4," e.g. "255.255.255.0."