Wi Fi extension
I have Airport Extreme. What device should I purchase to extend Wi Fi range?
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If you intend to extend wireless by wireless then by far the best is another airport Extreme.
If you can manage to run ethernet then any wireless router with AP mode or wireless access point can be used.
Since Apple no longer make airports.. for more than 1 year now.. you might have to buy something else before too long anyway. It could be a good time to move to another brand.