Operation
ICS routes TCP/IP packets from a small LAN to the Internet. ICS maps individual IP addresses of local computers to unused port numbers in the TCP/IP stack. Due to the nature of the NAT, IP addresses on the local computer are not visible on the Internet. All packets leaving or entering the LAN are sent from or to the IP address of the external adapter on the ICS host computer.
On the host computer the shared connection is made available to other computers by enabling ICS in Network Connections, and other computers that will connect to and use the shared connection. Therefore, ICS requires at least two network connections. Normally ICS is used when there are several network interface cards installed on the host. In special cases, only one network interface card is required and other connections may be logical. For example, the host may connect to Internet using a modem/router configured in the bridge mode and share the PPPoE connection with ICS.
Starting with Windows XP, there are some improvements to ICS. Internet Connection Sharing is integrated with UPnP, allowing remote discovery and control of the ICS host. It also has a Quality of Service Packet Scheduler component. When an ICS client is on a relatively fast network and the ICS host is connected to the internet through a slow link, Windows may incorrectly calculcate the optimal TCP receive window size based on the speed of the link between the client and the ICS host, potentially affecting traffic from the sender adversely. The ICS QoS component sets the TCP receive window size to the same as it would be if the receiver were directly connected to the slow link.
Internet Connection Sharing also includes a local DNS resolver in Windows XP to provide name resolution for all network clients on the home network, including non-Windows-based network devices. ICS is also location-aware, that is, when connected to a domain, the computer can have a Group Policy to restrict the use of ICS but when at home, ICS can be enabled.
Read more about this topic: Internet Connection Sharing
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