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My ISP game me a device that offers hardly any configuration possibilities. The Wi-Fi is also bad. So I decided to buy another router and flashed it with OpenWrt.

Unfortunately, I cannot replace my modem because otherwise I am not able to connect to the internet.

What I achieved so far:

enter image description here

OpenWrt Router is running a DHCP Server and gives out IP addresses within 192.168.1.1/24. This works well for Device 0, Device 1, as well as Device X. Every device can connect to the internet and ping each other.

When I connect Device 2 to my modem it also receives an IP within 192.168.1/24. It looks like that it does not found any route to my OpenWrt router, or at least it does not know that it could use the IP Address 192.168.0.2 as a gateway to the other subnet. Please remember there is not much I can configure within my modem's settings.

Do you have any ideas on how to make it possible that Device 2 can connect to the internet as well as to connect to any other device? Should I put the OpenWrt into the same subnet? How would I configure the DHCP Server to use OpenWrt as Gateway/DNS for every device to be able to use the OpenWrt adblock feature, which is using a special local DNS as far as I understood? Or is it possible to use a bridge on OpenWrt Router side?

Update:

  1. I want to connect Device2 to my Modem since it is basically a Router and it has open ethernet Ports. I do not want to buy an additional Switch as long as I don't need to.
  2. My Modem is something propietary from Vodafone Germany for Cable (Docis 3.1) Access: enter image description here
  3. My OpenWRT Router is a TP-LINK Archer C7
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    As a heads up, this question belongs on SuperUser, because it relates to home networking. – Spooler Oct 21 at 22:33
  • I don't think your "modem" is dumb, since it's acting as a modem, router, wi-fi access point and probably switch (if it has more than one Ethernet ports); you simply don't have administrative access to it and can't change its config, but this does not mean it's "dumb" as you think. – Massimo Oct 21 at 22:49
  • Why the heck are you attempting to connect Device 2 directly to your modem? What do you think you get from doing that? That device should just be connected directly to the router. – Giacomo1968 Oct 22 at 1:15
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    Though we really do need to know more about this modem/router. A model number would be nice. – Spooler Oct 22 at 3:44
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    @spooler you are assuming facts not in evidence. If the wan of the dd-wrt box was connected to the modem that leaves device 2's IP address unexplained (but your interpretation is supported - weekly - by the diagram). Similarly "using a bridge" may depict a willingness to give up some dd-wrt functionality. Until we know why device 2 is not behind dd-wrt and can deduce the logic behind how they want it to work - which the cabling will hint at - I don't think we can accurately answer. – davidgo Oct 22 at 3:56
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There are a few things going on here. I'll summarize.

That "modem" is much more than a modem. It's a router, DHCP server, DNS server, gateway (to the modem) and an L2 bridge (switch). We'll call that network device R0.

The OpenWRT device (we'll call it R1) is also all of those things, with the exception that its gateway route points to R0's private L2/3 domain (192.168.0.0/24). You created 192.168.1.0/24 yourself outside of the scope of R0, and did not configure R0 to route to that subnet. R1 is also configured as a NAT gateway for that subnet, which further obscures it from 0.0. The configuration you've rendered has effectively isolated 1.0 from 0.0, as if 0.0 is a public subnet, and 1.0 is a private subnet. 1.0 can get to resources on 0.0, and can use 0.1 as a gateway to the internet, but 0.0 cannot get to 1.0 without configuring the R1 NAT to do so (via port forwarding or similar). This is referred to as a "double NAT".

This brings up a question: Why is device 2 connected directly to R0? This connectivity makes use of both network domains, and I'm not confident that you actually want two network domains based on your question.

And finally, I would suggest a few things to remedy this:

  1. Use R1 as a bridge, not as a router/gateway. This would make OpenWRT act as an L2 switch, placing all devices on a single L2/L3 domain (192.168.0.0/24) that the modem/router serves network services for (DHCP, DNS, and routing). This is the simplest solution, and I imagine probably what you're going for here. There would be no 192.168.1.0/24.
  2. Configure R0 to act as a bridge, not as a router/gateway. This one is trickier, because it involves configuring this modem/router. There is probably a way to do that, though I'm not certain without you providing a model number. This way, the modem would be ONLY a modem and L2 bridge, and would only support a single device connecting to it - R1. R1 would get a public internet-facing address on its gateway NIC (provided via R0), and would be responsible for forming a NAT with that public IP and a private subnet (it could be anything you want, but let's keep it from possibly colliding if R0 gets reset and use 192.168.1.1/24). Device 2 would not connect to R0, but instead to R1. If you want to accomplish some kind of DMZ for device 2, you can do so via OpenWRT in R1.
  3. Get a different modem. Depending on what kind of fabric you're using, this can be pretty trivial. Don't put up with an unruly device in your network that makes you jump through a bunch of hoops. It sounds like the hardware in it isn't great anyways. This is what I would do, in conjunction with option 2. If it's something like a cable modem, the surfboard models are good. I would avoid combination devices, and get JUST a modem that presents ethernet on a single RJ45 jack.
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  • “Why is device 2 connected directly to R0?” The core question I have as well. What is the benefit of that for the end user? – Giacomo1968 Oct 22 at 1:16
  • The answer is simple: It has 4 Ethernet Ports and I do not want to add an additional switch to my setup. It is perfectly fine for me to use 0.0/24 for the whole network but I am struggling with the configuration. I can disable the DHCP Server from R0 and run it on R1. But which device would be the gateway to the internet and which one provides the DNS? And how do I configure the DHCP Server so the right information is distributed to the clients? I want to use the R1 DNS because of the integrated adblock plugin. Thank you. – caiuspb Oct 22 at 10:53
  • Is it possible to go with your Option 1, configure R1 to do DNS and DHCP and still have support for Upnp via R0? – caiuspb Nov 10 at 11:34
  • If you configure R1 to serve those services on its bridge IP, and turn those services on R0 off, then yes. If the L2/L3 domain is flat, feel free to have either network node serve these things in any mixture you want - but don't run overlapping services on both nodes unless they're configured to be clustered. – Spooler Nov 10 at 17:55

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