This one is very simple: if the subnet mask specified on the host does not match the subnet mask for the network, then communicating with the network will not be possible. Finding the subnet mask is usually quite easy. In recent versions of Windows, such information can be obtained by navigating to Settings | Network Connections, right-clicking on the network adapter you are using, and selecting Properties. There should be a list box displaying the installed protocols; scroll down to internet Protocol Version 6 or Internet Protocol Version 4 depending on what version you are using, and click on the Properties button. This should show you what the subnet mask is.

In Linux, finding the subnet mask is just as easy. For example, in Ubuntu or Mint Linux, click on the networking icon in the tray on the right end of the taskbar (the icon should look like two interconnected cables if you have a wired connection or a series of arcs if you have a wireless connection). This should launch the Network Connections dialog box. Find your connection (for example, Wired Connection 1) and double-click on it. This will launch the Editing dialog box, where you can change settings for the connection. Click on either the IPv4 Settings or IPv6 Settings tab to find the subnet mask.

Correct configuration of the subnet mask should allow intra-network communications, but a missing or misconfigured gateway setting will prevent a host from communicating with other networks. You should confirm that the gateway is set correctly, and since the gateway is often specified in the same place as the subnet mask, this should not be difficult to do.