Circular dielectric cavities are key components for the construction of optic microresonators and microlasers. They are one of very few cases where the transcendental equations for complex eigenmodes (resonances) of an open system (dielectric cavity) can be found analytically in an exact manner. The behaviour of those eigenvalues in the small opening limit, i.e. when the refractive index of the cavity goes to infinity, is analysed. The analysis allows one to clearly distinguish between internal (Feshbach) and external (shape) resonance modes for both TM and TE polarizations. As a result, unambiguous azimuthal and radial modal indices are assigned to each internal and external resonance mode.