You will need 3 network cards, 2 for the main computer (1 for modem and 1 for switch/hub/computer) and another one for your client.
One computer acts as a server. This computer has two NIC cards in it. One is connected to the modem and the other is connected to your switch/hub/computer.
The main computer must be turned on if the other wants access to the internet. The main computer will have your connection software on it, and you'll use that to connect. The second computer has a NIC card installed and is connected to the switch/hub/computer. You can try Internet Connection Sharing supplied with Windows, or sharing software like AllAboard, Sygate, or Wingate.
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Buy a router. Each computer needs just one NIC card. You connect the modem and computers to the router. Set your router to "Obtain an IP address automatically" and it will connect for you. Both computers can use the connection independently of each other.
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You can use a hub, but will need a subscription to a Comcast Business account to get additional IP addresses.
*This FAQ is based on user knowledge from a volunteer core of BroadbandReports' members. This FAQ in no way constitutes official information from Comcast or any of its affiliates.