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Advanced Networking for Mactintosh Professionals


User Guide


Connecting to the Internet Using a One Way Cable Modem

Setting Up The Internet Connection

One way or Telco Return (TR) cable modems use the cable system as their downstream connection but require a conventional dial-up PPP modem connection for outgoing data (upstream). This page offers a brief overview of how to configure IPNetRouter (TCP/IP) for use with these systems.

To connect a single host through a TR cable modem, you need multihoming so you can have both a PPP and Ethernet IP interface active at the same time. This is the same as connecting via PPP, except your Ethernet Interface is now being used for the downstream link. What IP address to use for your Ethernet interface is determined by your cable modem provider. Some use, others use .

Getting a single host to work through a TR cable modem is the first step. Once this is working, the next step is to route this connection to the rest of your LAN. To get a single host to work through a TR cable modem, you can follow the instructions for Connecting to the Internet using OT/PPP and Ethernet except that your first Ethernet interface will be used to connect with your cable modem instead of your LAN. Here is a quick review of the steps required:

Run the IPNetRouter installer on the actual machine you wish to use it on. Be sure to use the correct version for your processor (PPC or 68K). The installer will place a copy of the IPNetRouter application on your hard drive, and a shared library named "OTModl$Proxy" in your Extensions Folder.

Next you must configure the TCP/IP control panel. You will need to create two separate TCP/IP configurations: one for your Ethernet; and a second for PPP (Remote Access).

Your Ethernet configuration will probably be set to "Configure Manually" so that you need to specify an IP address, Subnet Mask, Router Address, and Name Server Address. I can't tell you what values to use for these since they depend on your Internet Service Provider. You will need to get this information from your cable modem ISP.

Next you will need to create a PPP (Remote Access) configuration. Press Cmd-K to bring up the configurations dialog, and create a PPP configuration named "IPNetRouter" (the name is important). This configuration may be set to "Configure via: PPP Server", but you will still need to specify a Name Server Address as provided by your ISP. If you have not connected via PPP (Remote Access) before and are not sure how this works, you may need to test this configuration.

Once your PPP configuration is working, set the User Mode (under the Edit menu) in the TCP/IP Control Panel to Advanced, Choose Options, and uncheck "Load only when needed" for both your PPP and Ethernet configurations. The last step before finishing with the TCP/IP Control Panel, is to make Ethernet the primary configuration (the visible one) in the TCP/IP control panel.

At this point, you are ready to test that your single host can successfully connect to the Internet using your TR cable modem. Launch IPNetRouter, in the Interfaces Window select PPP in the PortName popup menu, check "IP Masquerading", and press "Add". IPNetRouter should invoke PPP (Remote Access) to dial out, connect, and bring up a PPP interface in addition to the Ethernet interface you made active from the TCP/IP Control Panel. Try using an Internet application like your web browser to verify whether this is working (if not, see the Troubleshooting Tips and other instructions).

To do this, you will create another IP interface on the same or another physical Ethernet to communicate with the rest of your LAN (as described in the instructions for a conventional 2-way cable modem). Of course this interface needs to have a different network number from the interface used to receive data from the cable modem. If your first Ethernet interface uses an IP Address in the range 192.168.0.x, you can't use an addresses from this range for your second Ethernet interface. You could use 192.168.100.x instead for example.

Up to this point, we have simply combined the instructions for using PPP and using a cable modem. Since we are sending upstream data via the PPP interface, we enable IP masquerading on this interface. If you want to setup your IPNetRouter connection for Internet sharing, more instruction is below. Do not proceed further if you do not have basic connectivity on the gateway.

Internet Sharing with One-Way Cablemodem

The tricky part is that we need to reverse the NAT (Network Address Translation) process for packets received on the first Ethernet interface as opposed to the PPP interface where masquerading has been enabled. IPNetRouter provides a switch called "TR Cable Modem" for this purpose (refer to the help text for the Gateway Window). It enables NAT for packets received on other interfaces.

Notice you may need to restart once to give Open Transport a chance to build your Ethernet link stream with the supplied OTModl$Proxy shared library. I recommend that you install IPNetMonitor, available from our website, and verify that you can monitor IP traffic on your Ethernet interface using the Monitor Window. This insures OTModl$Proxy has been successfully inserted into your Ethernet link stream.

The last step is to configure any hosts on your LAN to use the machine running IPNetRouter as their Internet Gateway. You do this by giving them IP Addresses in the same network range as the second Ethernet IP interface you created above, and setting their Router Address to the IP Address of your second Ethernet interface on your gateway. You will also need to specify an appropriate Subnet Mask and Name Server Address for these machines (use the same Name Server Address as for your gateway machine that should have been given to you by your ISP).

That's it. You are ready to enjoy surfing the web from any machine on your LAN using your One Way Cable modem.

Peter Sichel
April 12, 1999