How would I go about connecting a modem to a router when they use different subnets?

xanado · Apr 14, 2015

Hello. I have a VDSL modem provided by my ISP, and a router that forms the backbone of my network.
With my old ISP and the old modem I used with them, I'd just plug the modem into the WAN port of the router,
and because my router's gateway IP is 10.0.0.1, I'd set it's DHCP range to be 10.0.0.2 - 10.0.0.253, and on the modem I'd set it's IP to be 10.0.0.254 to avoid potential clashes. Doing it this way I could access the modem's admin gui through the network, and all computers had access to the Internet. All was well. Until I had to changed ISP and the old modem no longer worked. They sent me their own modem and I have come up against a pretty big problem.

For reasons I won't go into, my LAN has to be on a 10.0.0.x subnet. And this new modem does not allow that subnet to be used (this is by design, no a flaw in the software).

So, now I have the modem connected to the router like the old one, via the Router's WAN port. But the new modem's IP is 192.168.1.1 and my Router is still 10.0.0.1.

Can anyone explain to me if there is a way I can setup the router so it can "see" the modem so that I can access the modem's web gui from the network for example and all computers can access the Internet?

I know the simplest solution is to change the LAN subnet to match the modem but, as I say, I can't do this for various "mission critical" reasons.

hang-the-9 · Apr 14, 2015

Setup a network bridge http://windows.microsoft.com/en-us/windows-vista/what-is-a-network-bridge http://windows.microsoft.com/en-us/windows/create-network-bridge#1TC=windows-7

You can do that either with software or a hardware router with that capabilities.

You can also setup a proxy system where the proxy computer has two network cards, one connecting to the internal 10.x.x.x network, the other to the modem network and will route internet traffic to that. If you do a web search on how to setup a proxy server you will find a lot of info, you can use any cheap computer with a free Linux setup for that.

xanado · Apr 14, 2015

Thanks for the suggestion but, I still couldn't make heads or tails of how to do it and I've got a feeling it cna't be done with this modem the way I need to. Bridging via a PC or using a separate proxy system was not option unfortunately, so if I was to bridge It'd have to be done via the router and only the router. If any kind of setting needs changeing beyond the IP addresses on the modem's end though that would have instantly ruled that possibility out anyway because the modem is a rather simplistic "home network" kind of thing and lacks any powerful features like, routing, bridging etc. All it allows me to do is turn DHCP on and off, change the IP ranges for DHCP and the gateway IP but beyond that it doesn't really offer any other settings that would help in this case.

I spent hours googling for answers and I've come across a multitude of solutions all ranging from static routes to bridging but as I say I don't really understand the terminology enough to be able to do it for my hardware. There was talk of static routing for instance, I fiddled around with the static routing options of my router for an hour and couldn't get it to work.

I give up. I am instead, going to obtain another modem that works with my ISP and allows me to change the IP address to the 10.x.x.x range, it's the simplest solution. I only wanted to try and find a way to do it the way I wanted to mostly out of curiosity if it could be done and because it would save me the hassle of sourcing a new modem that actually works on my new ISP's line.

If though anyone can figure out a way around this particular problem then by all means explain it here because even though I'm still going to get a differetn modem, I would still like to know if it could be done this way (you never know, someone else in my shoes might come along).

hang-the-9 · Apr 15, 2015

To do things with a router or switch you need a smart one that you can go into and configure the ports, most home devices do not let you do that.

Why can't you setup a cheap PC or laptop to sit and act as the gateway and bridge? You can get a used dual core laptop with 4 gig of RAM for about $100, and an add-on card for the same for maybe 10-20. Will be the same as getting a new router or modem. A small laptop won't use much electricity even if on 24/7 and it has a screen and keyboard built in so you don't have to keep stuff sitting and getting in the way.

Plus you'll learn how to setup a proxy/gateway server as a bonus

You will need to mess with static IPs though, you will have the proxy system on the IP of the modem, then the second card on the IP of the computers and set the computers to use the 10.x.x.x address, manually set the gateway IP as the IP of the laptop that is the proxy, and set the proxy IP of that computer in the web browsers.

First network card to modem/router to the proxy computer, then the second network card to the switch that is connecting your other computers. The proxy server will talk to the modem and IPS for internet connection, the other computers will talk to that proxy computer and get the data passed to them from it.

Urumiko · Apr 15, 2015

If it is a true router you should have no problem in setting it's interface to be on the same network as the modem while keeping your router and all of its other interfaces on 10.x.x.x.x. after all this is what it would do to connect to the internet.

interfaces facing each other should always be in the same subnet, this is a fundemental rule of how networks operate. The only way I can think of working around it is to have a device inbetween like another router or firewall.

Can you explain a bit more about why you beleive you cant use the modems subnet?

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How would I go about connecting a modem to a router when they use different subnets?

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