When a Windows 2000 ICS client quits the program that established a connection through the shared modem of an ICS host, the connection does NOT automatically disconnect.
This behavior is by design. With ICS, you cannot configure a modem to automatically disconnect when you quit a program on the client computer that uses the connection.
To workaround this behavior, on the ICS host:
1. Start / Settings / Network and Dial-Up Connections.
2. Right-click the connection and press Properties.
3. Select the Options tab.
4. Change the Idle time before hanging up box (under Redialing options) to a time to wait before before the connection disconnects.