Subnet mask you can find out by looking at settings, network connections. Go to the start menu on the toolbar. Find the setup tab, select network connections. Before us there is a network connections window. If you have a network connection, in this window, they should be displayed. If the window is empty, then you need to create a network connection. Now consider the variant when you have all created.
Right-click on the network connections icon. In the appeared menu select the item "properties".
Appears properties window. Find a window with the title "components used when connecting".
Move the slider down window, find "Internet Protocol (TCP/IP)". Click with the left mouse button on the item. Just below become active-click on "properties".
Click on this button, a window "Properties: the Internet Protocol (TCP/IP)". Where you will see the IP address of your computer and the mask for the network. Typically, the mask value is always the same: 255.255.255.0.
If you first configure a network connection, Properties Internet Protocol (TCP/IP)" once you ordered an IP address, click with the left mouse button in the field subnet mask and 255.255.255.0 as the subnet mask appears automatically.