As a follow-up... If you're talking about a place that has existing wiring...

When you have two pairs in the wall like that, you often are talking about a system where all the jacks are wired together for two phone lines only. In that case, you can't run ethernet across those wires. You can't plug multiple ethernet devices into the same wire-stream without a hub to negotiate among them. If that's the case, you could look into HPNA cards...