. . will be provided by your cable company. The Linksys does not do that conversion but is actually just an ethernet router with no cable or DSL smarts whatever. You are concerned with what's in Peter's blue box where the Linksys would replace his Buffalo. Outside the box is provided by the cable company.
The Linksys (or any similar unit) has one WAN ethernet connection that can plug into any ethernet device. It has 4 LAN ethernet connections and the wireless access point which connects internally.
The Linksys is a Linux box which provides basic firewalling, port forwarding and that sort of stuff on a scale suitable for home or small business.