FFT of High Frequency and Low Frequency?
The frequency span of an FFT is determined by the number of samples fed into it and the sample rate of those samples.  Basically, the maximum frequency analyzed by the FFT will be half the sample rate.  The lowest frequency will be DC, and the remaining output "buckets" will be evenly spaced in frequency up to the (last) max frequency bucket.  The number of output "buckets", in the standard implementation, is half the number of input samples.

(Actually, the total number of outputs is equal to the number of input samples, but half the outputs are the imaginary half of complex numbers ... and at about this point my head starts to hurt.)