Clarification regarding glClipPlane

I need some clarification regarding glClipPlane. Lets say the I define an additional clipping plane between zNear and zFar. Then which part of the frustum gets clipped? The one between zNear and clip plane or the one between zFar and clip plane?

What I want to do is clip the front and back of an object. When I define 2 clipping planes (z=c1, z=c2) to do this, I see only one part of the object outside the 2 planes and not the part between the 2 planes.
While what I want to see is the part of the object between z=c1 and z=c2. Can this be done using clipping planes?

Planes have direction which defines the clip halfspace.

This direction is defined by the first 3 coefficients of the plane equation which could be considered the surface normal of the plane you are defining.