Do you have "Cable" broadband? The problem sounds as though you have connected one of a router's LAN ports by ethernet to a Cable Modem, so that it is functioning as a Network Switch, not a Router, and therefore passing the addresses allocated by the ISP to the first computer to connect.
With one computer connected, open a Command Prompt window ("Start > Run" type CMD and click OK) and run the command IPCONFIG /ALL which will show details of all network adapters in the PC. With a Belkin Router, It should have an IP address starting with 192.168.2 and with the final number between 2 and 254. The Subnet Mask address will be 255.255.255.0 and the Default Gateway should be 192.168.2.1
A friend of mine had this problem and was initially told by his isp that only 1 could connect at a time. When he said this was not good enough and give me a mac to leave - all of a sudden he was transferred to a tech who knew what he was talking about. After a few instructions from the guy and I think an alteration at the far end - low and behold any number of units can connect at the same time.
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