Internet through wireless hotspot, intranet through wired network - can I use both at the same time?

Question

I have been tring this for a while now, without success. I have read online this can be done, either by changing the default gateway (DG) or messing with the DG's metric

The short story is: I need to connect to my company's ERP system (port 80 for some web services and port 22 for sftp and bash) with using my notebook computer's LAN card - but I can't connect to the Internet on this network. So I have my mobile phone's 3g hotspot when I need to go online. But I want to use both at the same time without so much as opening another web browser process.

At the moment, I am using the ERP system regularly - and whenever I need to go online to check a mail or research something related to work, I need to unplug the network cable so the wifi can kick in... which is starting to become annoying especially if I am connected to the ERP system using FTP (Filezilla) or Shell (putty)

Can someone help me with the configurations?

Below are some of the details I can provide:

Wired network - office LAN - for connection to the ERP system:
DHCP -    Yes
IPv4      10.64.200.245
Subnet    255.255.240.0
DG        10.64.193.254
DHCP Serv 10.64.17.191
DNS       10.68.72.200
          10.68.20.200
WINS      10.68.77.200
          10.64.20.200

Below are the details for my WiFi internet connection - over my mobile hotspot

DHCP      Yes
IPv4      192.168.43.39 <- I'm guessing this changes every time I restart the wifi
subnet    255.255.255.0
DG        192.168.43.1
DHCP      192.168.43.1
DNS       192.168.43.1

Finally, here is the tracert results (btw, I'm on Windows 7, 64bit and the remote system should be on Linux) - the target ERP ip address is 10.76.142.39

Tracing route to 10.76.142.39 over a maximum of ...

  1    <1 ms    <1 ms    <1 ms  10.64.193.253
  2    <1 ms    <1 ms    <1 ms  10.64.4.13
  3    <1 ms    <1 ms    <1 ms  10.76.23.124
  4     1 ms    <1 ms    <1 ms  10.76.23.93
  5     1 ms    <1 ms    <1 ms  2xx-xx-xx-9.adsl...
  6     1 ms     1 ms     1 ms  2xx-xx-xx-9.adsl...
  7     1 ms     1 ms     1 ms  2xx-xx-xx-10.adsl..
  8     2 ms     1 ms     2 ms  10.76.135.125
  9     1 ms     1 ms     1 ms  10.76.142.39

sorry, I had to mask out some details above... wasn't sure if they would have any confidentiality issues.

Any help is much appreciated.

Answer 1

A default gateway means (simplistically) "If I don't have an explicit route for this network, send it to the default gateway".

It is the route that is taken by default. However, you can have specific routes for networks where you know how to get to them.

In your case, for any internet based destinations, which could be anything, you want to go via the hotspot, so your default gateway should be 192.168.43.1 on the wifi interface.

The wired interface should not have a default gateway at all. We know the destinations we want to access via this interface, so we can add specific routes. The next hop on that network is 10.64.193.254, so for any networks we want to get to internally, we can have a specific route to get to them.

By the look of it, they use the 10.x.x.x network for everything. So you can go to cmd and do:

route add 10.0.0.0 mask 255.0.0.0 10.64.193.254

And remove the default gateway from this interface.

So what we are saying is "If the destination network is in the 10.x.x.x range, send the traffic to 10.64.193.254 out of the wired interface, otherwise send it to 192.168.43.1 out of the wireless interface."

As 10.76.143.39 falls into the 10.0.0.0/255.0.0.0 network range, it will take the specific route added above.

Something like 8.8.8.8 does not fall into this range, and so will take the default route out of 192.168.43.1.