The purpose of the subnet mask in the "ethernet" world is to limit or restrict the IP addresses allowed on a part of the network or "subnet". In other words, it limits how far from the units IP address the device attempting to "talk" to the unit may be. For example, a subnet mask of 255.255.255.240 limits the valid address range to 16. If your PC's IP address is off by more then 16 from that of the RTU, they will not be able to "talk" to each other. FYI - There are calculators available for free on the internet that can be used to calculate the subnet mask range.