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Cable Modem vs. Dial-up

Sometimes dial-up Internet connections must be used, even when you have broadband. Here's how to dial-in without losing your high-speed access to the Internet.

By Joe and Ron of Neighborhood Techs

Q. My computer has a network card which is connected to a cable modem. However, I must use a 56K modem to connect to the network at work (they don't have a VPN in place).

Whenever I use the modem to dial-in, I lose my broadband connection. When I disconnect the dial-up session, the broadband connection is automatically restored. Why is this and can I prevent the NIC from dropping my connection when the modem is in use?

A. This is a common problem when both a network and dial-up connection are in use on the same machine. When you connect to your corporate network, your NIC's connection to the Internet via the cable modem isn't actually lost. It just looks that way.

Every computer with a network connection (such as a connection to the Internet) has a route table with a default route, which sends traffic addressed beyond your LAN to the default gateway, which in this case is the cable modem (or the router connected to it, if applicable).

When you initiate a dial up connection, the default gateway, which was the cable modem connection, is replaced by the remote access server so the dial-up becomes the gateway. Now any outgoing traffic will go in and out of the dial-up interface instead of though your cable modem connection.

You can see this in action by viewing your route table in each scenario. At a command prompt (in Windows, go to the Start menu, select Run, and at the Run dialog box, type CMD), type ROUTE PRINT while the modem connection is inactive, and again after connecting. When the modem is active, you'll see an additional ROUTE statement at the top of the list.

You didn't indicate what operating system you were running, and different versions of Windows deal with the default gateway issue differently, but here are a couple of things you can try to resolve the issue.

In the properties screen for your dial-up connection, find the setting labeled 'use default gateway on remote network" and clear it. In some cases though, this may prevent you from accessing resources on the corporate network. If this occurs, or if it doesn't solve the problem, you can manually add explicit routes to your computer's route table. Explicit routes override default ones, so they should direct traffic back to the cable modem.

The proper syntax for setting up an explicit route at the command prompt is to type in the format:

ROUTE ADD <network address> MASK <subnet mask> <gateway address>

So, in this case, you should actually type the following:

ROUTE ADD 0.0.0.0 MASK 0.0.0.0 x.x.x.x

In this case, x.x.x.x is the IP address of your router or cable modem.

Then type:

ROUTE ADD a.a.a.a MASK b.b.b.b c.c.c.c

This second time, the "a" and "b" addresses correspond to your corporate subnet and mask (for example, 172.16.0.0 and 255.255.0.0 -- double check these with your network administrator at work). The "c" address is the IP address your dial-up adapter gets when connecting.

The first line restores your default gateway, sending all unspecified traffic out through your cable modem. The second line makes sure that attempts to reach your corporate network are correctly routed to the dial-up interface.

Adding these routes should allow you to simultaneously access your corporate network while still using the preferred cable modem for other forms of traffic.

Here is the tricky part, though-- if your dial-up connection is assigned an IP address via DHCP (and many are), then the "c" address will change each time you connect. You'll need to re-enter the route commands with the new "c" address each time you connect via dial-up.

Advanced users can save a lot of typing by creating a batch file and running it whenever needed. If you're using Windows 95/98/ME, any additional routes you add will be lost whenever you shut down the system, anyway. If you're using a static address for your dial-up connection and you're running Windows 2000 or later, make the route statements persistent by adding "--p" (without the quotes) to the end of each line.

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