Sharing a DSL connection between two or more computers is best done by using a router. A router is a device that connects two or more computer networks. In the case of sharing a DSL connection the router connects the Local Area Network (LAN) of your home or business to the Wide Area Network (WAN) that is the RTC on Line network connection to the Internet. Each computer will need an Ethernet Network Interface Card to interface with the router.
Router
Information:
There are many Cable/DSL routers available for home and small office use.
Below there is some information on configuring a Linksys Cable/DSL router.
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Linksys Cable/DSL Router
Configuration
Linksys Cable/DSL
Router