An ideal filter is one which

Ideal filter

An ideal filter is one that has zero attenuation in the pass band and infinite attenuation in the stop band.

The characteristic of an ideal filter is shown below:

Here, f_c1 and f_c2 are known as the lower and upper cut-off frequencies respectively.

An ideal filter will pass the frequencies lying between fc1 and fc2 and block the frequencies lying outside fc1 and fc2