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I have a friend who works in a small office with ~10 computers + 4 servers. Local service provider told him that there will be a planned maintenance and they will have ~2-3 days of internet downtime.

He asked me to help him about this. They already have a usb 3g modem, which emulated as a modem in local pc. That specific pc is able to connect through 3g, but they want to share it across the network.

They have around 10 pcs and 10 servers, all hooked up to an unmanaged switch. Switch is hooked up to a tilgin ADSL modem, which also acts as a router and dhcp server (a pretty common setup actually). ADSL modem supports static routing.

I was going to tell him to set up the default gateway for all pc's as the pc with the 3g modem, but it requires the ip's to be assigned staticly. This will render routing by pc name useless, and thats an important functionality for them since they all require to access the routers and other pc's by their network names.

Can I setup a routing in the present adsl modem to route all web requests to that specific pc hooked up with 3g modem, while all other local lan requests works as they do now? As far as I can tell, they theorically can do this, but I don't have enough experience to setup the rules.

So can it be done? If so, how should I setup the rules?

SercioSoydanov
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  • You haven't mentioned the exact model but if the modem/router has a USB port you might be able to plug the 3G dongle into that and actually leave the current settings in place. – Burgi Dec 15 '15 at 14:37
  • You should also mention to your friend that their bandwidth will be extremely restricted on 3G. They might want to consider limiting connectivity to business critical activities only (external email and VoIP) – Burgi Dec 15 '15 at 14:39
  • @Burgi that is a great idea, I just asked him through e-mail. By the way I already told him that it will be like a turtle, but they just want to be able to send plaintext emails at least. They are not using VoIP anyway. – SercioSoydanov Dec 15 '15 at 14:43
  • @Burgi modem is a Tilgin HG1332 and it indeed has a USB port. We will try it out tomorrow. Thanks for the idea :) – SercioSoydanov Dec 15 '15 at 14:54
  • No worries, let us know how it goes – Burgi Dec 15 '15 at 15:16

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If this specific computer with the 3G modem runs with Linux, you can easily activate ip forwarding and tell all computers to use this Linux machine as the default gateway.

I dont know how to solve this with a Windows machine, but you could try to use this 3G Modem as a gateway through your specific ADSL Router. You have to tell the ADSL Router to use port x as the Internet Connection an behind this port there is the 3G Modem. Something like "Internet through LAN PORT 1".

Artur
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  • I am so jealous of people who don't know how to do things on windows world :) (no sarcasm intended). 3g modem is indeed going to be hooked up to a windows machine. Lucky for me, for this specific event, I do know how to share the 3g connection. My main problem is to route all network traffic to this specific machine. In my case, setting each other pc's ip settings is not feasable, so if you could just type me the forwarding rule to set up in the router, that would be fantastic - If the router don't have usb port, as Burgi above suggested. – – SercioSoydanov Dec 15 '15 at 14:46