If the subnet mask specified in a node's network settings does not match the subnet mask for the network, then that node will not be able to communicate with the rest of the network. If there is a problem that only affects a certain node, and the node seems to be physically connected to the network, then this is definitely a possibility.

Finding the subnet mask of a node is usually an easy affair. If the node is running Windows, navigate to Settings | Network Connections, right-click on the current network adapter, and select Properties. There should be a list box displaying all the available protocols. Select Internet Protocol Version 6 or Internet Protocol Version 4, depending on which version is being used. When you have selected the right protocol, click on the Properties button, and a dialog box should appear displaying, among other things, the subnet mask.

You can find the subnet mask in Linux in a similar way. If you are running Ubuntu or Linux Mint, for example, double-click on the networking icon at the right end of the task bar (the icon should look like two connected cables for a wired connection and as a series of arcs for a wireless connection. You should be able to launch the Network Connections dialog box. In this box, you should be able to find your connection and double-click on it.

The Editing dialog box should appear. This box provides you with the ability to change settings. Click on either the IPv4 Settings or IPv6 Settings tab. Once you do this, you should be able to see the subnet mask. Make sure that the subnet mask matches whatever it should be for the subnet. For example, if you are on the 192.168.1.0/24 subnet, the mask should be 255.255.255.0.

The correct configuration of the subnet mask is a necessary precondition of network communications. However, not setting the gateway (or setting it to the wrong value) will prevent the node from communicating with other networks. Confirm that the gateway is set correctly. You can usually find this setting in the same place as the subnet mask.