Using Internet Connection Sharing for Backup Connectivity

This page contains notes on how to set up Internet Connection Sharing for backup connectivity. When your main cable/DSL modem connection goes down, you can connect through one of the PCs on your network that has a dial-up or alternate Internet connection.

Network Configuration

These notes apply assuming the particular network configuration given below. You have multiple PCs behind a router that is connected to the Internet via a DSL or Cable Modem. One of your PCs has an alternate Internet connection, such as dial-up modem or a laptop wireless data card. The advantage of this backup connectivity plan is that it requires no changes to the configuration of the individual computers on your network. Essentially, the PC with the backup connection takes the place of your dedicated router. That PC must be left on and connected to maintain connectivity.

Network Diagram

          Internet
              |
         Cable modem
              |
PC1  --|      |
PC2  --|--- Router (Wired/Wireless)
       |
       |-- PC3 -- Dialup --- Internet
       |--... PC4

Normal Router Configuration
    Internal (or Local, LAN) IP Address: 192.168.0.1
    External address: x.x.x.x as supplied by your ISP

Normal PC1/PC2/PC3 configuration
    DHCP or Fixed IP on 192.168.0.x subnet
    Gateway & DNS server is 192.168.0.1

How to turn on Internet Connection Sharing

These instructions apply to Windows XP Home or Professional with Service Pack 2.

1. Router
   1. Open the router's administrative interface, usually http://192.168.0.1/
   2. Change the router's internal/LAN IP address to an alternate, e.g. 192.168.0.2
2. PC3 (the one with the dial-up)
   1. open Network Connections from Control Panel
   2. Right-click the Dial-up connection and select Properties
   3. Click the "Advanced" tab
   4. Select "Allow other network users to connect through this computer's network connection"
   5. If a PC has a hosted service (e.g. FTP or web), then click the "Advanced Button" Add IP addresses and ports that should be enabled
   6. Click OK to apply the settings (The internal IP address will change to 192.168.0.1)
3. PC1, PC2, etc.
   1. There is no need to change any settings.
   2. However, YOU MUST WAIT APPROX 1-2 MINUTES BEFORE ICS WILL BE RECOGNIZED

Notes

• There is no need to disable the Windows Firewall on the computer hosting the shared connection.
• The Internet Connection Sharing interface on the "Advanced" tab of the dial-up connection properties will be blank if there is no other enabled network connection.
• There is no need to change adapter priority on the computer hosting the shared connection. (Adapter priority is the order in which network connections are accessed by network services, when more than one is available. This can be changed from "Control Panel"->"Network Connections"->"Advanced" menu->"Advanced Settings..." menu item-> "Adapters and Bindings" tab->"Connections" list box.)
• The external IP address of any services running on the internal network will have changed to the IP address of the dial-up connection.
• If a new computer (PC4) joins your home network, it will obtain its IP address configuration settings from the router. Because the router has a temporary IP address, PC4 will end up connecting through the router instead of the PC hosting ICS (PC3). To avoid this, allow PC4 to acquire an IP address from the router, then edit the IP address settings and change the "gateway" and "primary DNS" settings to be the address of PC3, which is 192.168.0.1 in this example. The same procedure will apply to any PC that, for some reason, has to renew its IP address.

To test if the Cable/DSL modem is working again

On any PC, open IP address configuration from the main network adapter (usually a LAN or High-Speed Adapter). Change the DNS and Gateway to the router's temporary IP address (in this example, 192.168.0.2). Wait a few minutes, then attempt to load a web site like http://www.google.com/.

To revert back to normal configuration

Uncheck the ICS checkbox on PC3.
Change the router's internal/LAN IP address back to default
WAIT A FEW MINUTES FOR SETTINGS TO TAKE EFFECT