fft - In case of Complex DFT spectrum, why the x axis range from N/2 to N point mean a negative frequency?

When we transform a complex signal into frequency signal by using a complex DFT,

The range from N/2 to N point on the X axis of the spectrum mean a negative frequency...

But i cant understand why it is..

In addition, we should not use the range over the N/2 point because of the aliasing issue. But i heard that the negative frequency region is used in case of complex dft spectrum of a complex signal.

Very confused..

Could you clearly explain it for me?

Thank you.