Adding A Subnet and/or VLAN

My company will be installing a new industrial control panel at a municipal water treatment plant. The panel will have several components (PLC, HMI, VFDs, etc.) connected via Ethernet. The panel will also contain a fully managed Ethernet switch with DHCP and VLAN capabilities (

For obvious reasons, the folks at the plant do not want the devices in this panel to take 5+ IP address from available pool, so they've asked that this panel be configured as a subnet. For example, if the primary network uses IP addresses 192.168.1.x, the devices in this panel should be addressed 192.168.2.X, and should be accessible to 192.168.1.X devices.

The primary reason for this is to increase the number of available IP addresses on the network and to keep Ethernet/IP control instruments (VFDs, meters) somewhat separate from the more intelligent TCP/IP devices in the system (PLCs and SCADA). What is the easiest way to add these additional IPs (subnet?) to the system without having to reconfigure every other device already on the network?

Some of the abbreviations I've uses may not be familiar to folks who aren't involved in industrial automation, so here is an explanation:
PLC: Programmable logic controller. Basically an industrial computer.
HMI: Human-Machine Interface. The monitor and keyboard/touchscreen that allows operators to monitor and control the system
VFD: Variable Frequency Drive. Used to vary the speed of 3-phase AC motors.
Ethernet/IP: Ethernet Industrial Protocol. A communication protocol for industrial devices that allows communication over a standard TCP/IP Ethernet network.
SCADA: Supervisory Control and Data Acquisition. The top level control and data collection system that ties together the various sub-systems in a plant. Usually server based software by companies like GE or Siemens.
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More about adding subnet vlan
  1. After some more research, I think that a subnet is not exactly what I want. Because my main goal is to add additional IPs and sntill maintain communication between the two networks, I think what I really want is to create a class B network from a class C network.

    I'd appreciate any comments on the issue.

  2. Why not add a router for the 192.168.2.X network and connect it to a port on the 192.168.1.X network router? This is basically what you're being asked to do. As long as the routers are configured properly, you can add up to 254 devices while requiring a single port on their router.
  3. I set up a test network like this in my office, and devices on the .2.X network were able to access the internet and all devices on the .1.X network, but it only worked in one direction. For example, a .2.X computer could ping a .1.X computer, but I could not use the a .1.X computer to ping the .2.X computer. It looked like my .1.X router was ignoring all .2.X address requests, even if I used a subnet mask of
  4. I presume that both routers have been configured properly, i.e., a route to the .2.x router has been added to the .1.x router and vice and versa. What's the default gateway for hosts on the .1.x network? The default gateway has to be aware of the .2.x network and that it's reachable through the .1.x router by specifying it's .1.x IP address. The subnet mask has to be (24 bits).
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