  We study the dependence of entropy [per lattice site] of six-vertex model on
boundary conditions. We start with lattices of finite size and then proceed to
thermodynamic limit. We argue that the six-vertex model with periodic,
anti-periodic and mixed boundary conditions produce the same free-energy in the
thermodynamic limit. We have found fixed boundary conditions such that the
entropy varies continously from zero to its value for periodic boundary
condition. We have also shown that the physical quantities of the six-vertex
model at the isotropic point does not change in the case of singular toroidal
boundary.
