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I have a router (Teltonika RUT) that is connected via its WAN port to a network (let's call it "Big"). The IP Space of the "Big" Network is 192.168.1.0/24 with the Gateway on 192.168.1.1. The router now creates a network ("Little") that also has to have the same IP Space, with the gateway also being on 192.168.1.1.

I know that this is very much not ideal, but there are some devices and services on both networks that need the same IP Space. To move either one of the networks to a different subnet is unfortunately not an option.

The "Big" Network effectively provides Internet for the "Little" network. Devices do not need to communicate in between those networks, the devices in the "Little" Network just need to be able to communicate with the Internet via the "Big" Network.

Is there some way to make this work?

Thanks in advance.

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    You need a couple of routers and do Outside Source NAT. This is some thig that can be done, but it should only be a temporary situation until one or both sides can be re-addressed. It is a common situation with companies that are acquired or merge. Each needs its own router connected to the other router, and they perform the outside source NAT instead of the more common inside source NAT.
    – Ron Maupin ♦
    Commented May 6 at 15:35
  • There are home/soho routers that do this out-of-the-box, but no on-topic "eneterprise" gear will allow this because they don't allow multiple interfaces in the same network. The workaround is a complicated VRF mess.
    – Ricky
    Commented May 6 at 22:20

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The basic premise is this:

You map all addresses from "Big" to a new, unambiguous subnet when accessed from "Little". You also map all addresses from "Little" to another new subnet when accessed from "Big". The mapping can be done with static NAT.

As an example, you map packets from or to Big within Little to 192.168.101.0/24, and packets from/to Little to 192.168.102.0/24.

The problem now is to get the router to translate e.g. source 192.168.1.1(Big)/destination 192.168.102.2 to source 192.168.101.1/destination 192.168.1.2(Little).

With some routers you can use schemes like VRF, multiple routing instances or tricks using loopback addresses for double translation. Often you need multiple, chained routers to translate in two or more steps (usually you'd translate the source first and then the destination).

I have no idea whether that Teltonika router can do it. Even if, that network will become very hard to debug. The reasonable step is to renumber one of the networks.

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  • The Router I am using actually does support VRF, tho I am not really sure yet how to set it up correctly. One addition: Devices from the "Big" Network do not need to contact Devices from the "Little" Network, and devices from the "Little" network just need to connect to the gateway of the "Big" Network (For Internet Access), but nothing else.
    – bgc
    Commented May 7 at 6:15

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