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Wi-Fi is described as reaching "only" about 150 feet, which is a rough estimate of the radius of older B and G devices. With an 802.11n base station, the distance can be much farther, although it varies by which band you choose.
But you can extend the covered area by adding more base stations with overlapping signals. As a Wi-Fi adapter in a laptop or handheld moves across overlapping areas, it can automatically switch base stations while maintaining a continuous network connection—as long as you've set the network up right.