We propose a method to describe analytically the ionic hydration patterns near sub-nanopores. It agrees well with molecular dynamics simulations, predicts the locations of the trapped water molecules, and captures the intrinsic and extrinsic features of the nanopores. Our method would open the way to designing and optimising controllable nanoionic devices with on-demand selective and conductive properties, finding applications in water desalination, energy harvesting, and DNA sequencing.