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Unusual Topology Problem - Question

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ignacio rossi

New Around Here
Hello everyone, this is my first post on this forum (long time lurker, first time poster)

(First of all, excuse my english :()

I've come across an unusual network problem at work and i havent figured out a good way to solve it. Though it might be interesting to post it here.

Some background:
We had to extract data from an industrial plc through good ol' modbus (port 502 tcp request). The only limitation it had is that the request had to be made from another device within the same LAN. So to deal with that we set up a router so requests go through a NAT. (In addition of some static routing).
So we ended up with something like this:

So far so good, right?

The problem is that now we have to extract data from another PLC in another network which is identical to the previous one (same ip, network and mask).

Is there a good way to route route requests to both devices at the same time?
Hi Ignacio,

I have no experience with PLCs so I'm unaware of any limitations they may have, but from the diagram and information you provided it looks like one solution might be to set up multiple VLANS on your core router, assign ip addresses to them, replace your NAT router with a layer-3 switch, then assign each vlan to a port on your switch. That is assuming its not possible to create ip subnets in the individual networks, and assuming that each PLC has a unique ip on the various /16 networks. Just a guess ….

It is best not to use over lapping networks. If it was me I would first change the IP networks so each location has their own network. Then join the networks.
Hello all, thank you for your responses. Phils solution might work. Going through my original post i've realised i didnt post the "now" diagram, so here it is:

Our desire is to be able to send TCP requests to both PLCs at the same time. I realize how unusual this might sound given that the easy solution is to simply change the second plc IP. The problem is that both Network 1 and Network 2 belong to symetrical systems managed by the company that build them (They are rather complex industrial machines, so i guess having totally identical systems is a good way to "stadarize" code, overall architecture, etc.)

I've come with a "poor man solution" to this, but havent tested it myself. I'd also like you hear your thoughs about it. I uses 3 routers.
Here it is:

The idea is that tcp requests start from our lan with port 502 for plc1 and 503 for plc 2, they get router by the static route and end up on the wan side of the router with the port forwarding.
Could a frankesteins monter like this work?

Again, thank you for your answers and your time.

EDIT: ive used draw.io to make the diagrams. Here is the link for the xml file in case you find it useful for posting a response (link: https://drive.google.com/drive/folders/1J4pzAZVN5IYbopa-H3VGgzhG3Y0fNTMr?usp=sharing)
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Don't use the same network for 2 different locations. Change the location networks so they are different then join the networks.
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