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Windows Networking/Home Networking with router & 3G


Hi Michael

I am trying to help my parents set up a simple home network so that they can share printers and files. Unfortunately their setup is quite unusual. Mom has a laptop and Dad has a desktop. They both have Windows 7. They have an oldish (4 year old) SMC ADSL router that is not connected to the internet (they cannot get a line installed). It doesn't have a USB port that would allow the insertion of a GSM/3G modem.

They access the internet using a GSM modem - USB-type 3G modem that takes a SIM, which they take turns to plug into their computer. The question is in 2 parts.

1) If I connect Dad to the SMC with an Ethernet cable, and Mom either with an Ethernet cable or wifi, they can 'see' each other's shared folders and printers, no problem.

But then they are unable to access the internet using their USB-3G modem, as the Windows machines are expecting to get internet via the SMC router. If I switch off the router, then they can connect to the web, but then obviously no file/printer sharing.

I have been through all the settings in the SMC and cannot figure out how to disable internet access, and turn it into a "dumb" router, so to speak. An option might be to just network them with a crossover cable? - but I haven't tried that, because I think Windows might still not know that this is not an internet connection.

The 3G modem gives the computer an IP address generated by the cellular provider, which is random each time, so there seems to be (I suppose) an IP address issue here.

2) Part 2, and the harder part in a way, is: can I set it up so that they can also *share* an internet connection? IOW, if the GSM modem is plugged into Dad's computer, and connected to the internet, and they are both connected via the SMC, then Mom can also get access to the internet via Dad's PC.

I know there are now routers that can accept USB/3G dongle-modems as a 'backup' option for when the ADSL line is down, but funds are tight at the moment, and I am hoping to get them connected with the gear they currently have.

Your advice would be greatly appreciated.


It sounds like you are allowing the router to act at the DHCP server for your network.  If doing that, the settings will be such that the devices will go to that router in order to access the Internet.  You could get around that problem by assigning static IP addresses to all of the devices and making the default gateway the address of the device that provides internet, i.e. the address of the 3G modem.  The problem with that, of course, is that the 3G modem does not have a static address, meaning you would need to change the gateway on each device every time you used it.  That would not be very convenient.  

Sometimes you can cheat and just use the IP address assigned by the 3G modem as a static address. If your ISP lets you get away with this, you could assign an IP address and gateway for Internet, and then assign a second IP address consistent with your local address for access to other devices on your local network.  As long as the default gateway points to the 3G modem's address, this should work.  

I'm assuming that you know how to do subnet masking for this solution.  If not, send a follow up and I can provide more details.

On that computer that is connected to the Internet, you could also install free internet proxy software like this:

This will allow other devices to connect to the Internet through the one that is directly connected to the Internet.

Obviously this would be much simpler with a router that could connect directly to the modem.  I know that costs some money, but it would be easier and run better.

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Michael Troy


I have a fair amount of experience in peer-to-peer networking. I can answer questions about hardware, networking software, protocol settings, etc. I have some client-server experience, but not a lot with Windows-based servers. I can also give some advice on home network security: VPN, firewalls, anti-virus, etc.


I am the Director of Information Systems for a large law firm which connects about 300 users over five offices via a wide area network. We use client-server, peer-to-peer, remote access, VPN, Internet, and proxy servers.

I also have a peer-to-peer network of computers at home, with file and print sharing, remote access, shared network storage, and shared Internet access with a firewall.

BA George Washington University JD University of Michigan

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