The subnet mask of this Net interface. The default value for this object is 0.0.0.0. If either the netConfigIPAddress or netConfigSubnetMask are 0.0.0.0, then when the device boots, it may use BOOTP to try to figure out what these values should be. If BOOTP fails, before the device can talk on the network, this value must be configured (e.g., through a terminal attached to the device). If BOOTP is used, care should be taken to not send BOOTP broadcasts too frequently and to eventually send very infrequently if no replies are received.