from the XP help file
Internet Connection Sharing overviewWith Internet Connection Sharing (ICS), you can connect computers on your home or small office network to the Internet using just one connection. For example, you have one computer that connects to the Internet by using a dial-up connection. When ICS is enabled on this computer, called the ICS host, other computers on the network connect to the Internet through this dial-up connection.
When you are setting up a home or small office network, it is recommended that you use the Network Setup Wizard in Windows XP Professional to enable Internet Connection Sharing. The Network Setup Wizard automatically provides all of the network settings you need to share one Internet connection with all the computers in your network.
After ICS is enabled, and you make sure that all of your computers can communicate with each other and they have Internet access, you can use programs such as Internet Explorer and Outlook Express as if they were directly connected to the Internet service provider (ISP). When a request to the Internet is made, the ICS host computer connects to the ISP and creates the connection so that other computers can reach a specified Web address or download e-mail. To test your network and Internet connection, see if you can share files between computers and that each computer can get to a Web address.
Internet Connection Sharing is intended for use in a network where the ICS host computer directs network communication between computers and the Internet. It is assumed that in a home or small office network, the ICS host computer has the only Internet connection. While other computers on the network may have modems to access the Internet, their primary connection is through the ICS host computer.
You might need to configure the Internet Connection Sharing computer to work properly across the Internet. Services that you provide must be configured so that Internet users can access them. For example, if you are hosting a Web server on your home or small office network, and want Internet users to be able to connect to it, you must configure the Web server service on the ICS host computer. For more information, see Service definitions overview.
You must enable ICS on the public connection of your home or small office network. If you have more than one network adapter installed on your computer, you need to choose the local area connection that communicates with the rest of the computers on your home or small office network. This is also called the private network connection. If there are two or more local area connections, when you enable ICS, you must do one of the following:
Select one connection to connect to the rest of the computers on your network. For information about how to choose the private connection, see Public and private network connections.
If you have two or more local area connections, and all of them connect to the rest of the computers on your network, you need to bridge the local area connections before you can select the connection to your home or small office network. If you choose to create a network bridge that includes all of the local area connections to your network, the bridge is automatically selected when you enable ICS. If you create a network bridge that does not include all of the local area connections to your network, you can still select the bridge as your private connection. For information about how to enable ICS, see Enable Internet Connection Sharing on a network connection. For information about how to bridge connections, see Bridge connections.
For information about the protocols, services, interfaces, and routes that are configured by enabling Internet Connection Sharing, see Internet Connection Sharing settings
Important
You should not use this feature in an existing network with Windows 2000 Server domain controllers, DNS servers, gateways, DHCP servers, or systems configured for static IP addresses.
The ICS host computer needs two network connections. The local area network connection, automatically created by installing a network adapter, connects to the computers on the home or small office network. The other connection, using a 56k modem, ISDN, DSL, or cable modem, connects the home or small office network to the Internet. You need to ensure that ICS is enabled on the connection that has the Internet connection. By doing this, the shared connection can connect your home or small office network to the Internet, and users outside your network are not at risk of receiving inappropriate addresses from your network.
When you enable Internet Connection Sharing, the local area network connection to the home or small office network is given a new static IP address and configuration. Consequently, TCP/IP connections established between any home or small office computer and the ICS host computer at the time of enabling ICS are lost and need to be reestablished.
Notes
You must be logged on as an administrator or a member of the Administrators group in order to complete this procedure. If your computer is connected to a network, network policy settings may also prevent you from completing this procedure.
If a VPN connection is created on the ICS host computer, and ICS is enabled on this connection, then all Internet traffic is routed to the corporate network and all of the computers in your home or small office network can access the corporate network. If ICS is not enabled on the VPN connection, other computers will not have access to the Internet or corporate network while a VPN connection is active on the ICS host computer. For more information about creating a virtual private network (VPN) connection, see To make a virtual private network (VPN) connection.
