What Is Internet Connection Sharing (ICS)?

Definition:

Internet Connection Sharing, or ICS, is a built-in feature of Windows computers (Windows 98, 2000, Me, and Vista) that allows multiple computers to connect to the Internet using one single Internet connection on one computer. It's a type of local area network (LAN) that uses a single computer as the gateway (or host) through which other devices connect to the Internet. Computers wired to the gateway computer or connecting to it wirelessly via an ad-hoc wireless network can use ICS.

Some of the features of Internet Connection Sharing include:

Allowing most any kind of device (including non-Windows and older Windows systems) to connect without having to install additional client software.
Support for all of the connected clients to use many different protocols, including VPN and Internet gaming.
Clients are automatically given an IP address and configured for DNS through the ICS computer.

In Windows 98 or Windows Me, ICS needed to be enabled or installed on the host computer from the Control Panel Add/Remove Programs (on the Windows Setup tab, double-click on Internet Tools, then select Internet Connection Sharing). Windows XP, Vista, and Windows 7 have this built-in already (look in the Local Area Connection properties for a setting under the Sharing tab to "Allow other network users to connect through this computer's internet connection").

Note: ICS requires the host computer to have a wired connection to a modem (e.g., DSL or cable modem) or an aircard or other mobile data modem, and the client computers either wired to your host computer or connecting to it via the host computer's free wireless adapter.

Learn how to use Internet Connection Sharing:

How to Share Internet Access on Windows XP
How to Share an Internet Connection on Vista
How to Share an Internet Connection on Windows 7
You can also Share Your Mac's Wired Internet Connection via Wi-Fi (doesn't use ICS, but is similar)

Examples: To share one Internet connection among several computers you can either use a router or, on Windows, enable Internet Connection Sharing so that other computers connect to one computer that has an Internet connection.