  Li and Luo [Phys. Rev. A 78 (2008), 024303] discovered a remarkable relation
between discord and entanglement. It establishes that all separable states can
be obtained via reduction of a classicaly-correlated state "living" in a space
of larger dimension. Starting from this result, we discuss here an optimal
classical extension of separable states and explore this notion for
low-dimensional systems. We find that the larger the dimension of the classical
extension, the larger the discord in the original separable state. Further, we
analyze separable states of maximum discord of two qubits, and their associated
classical extensions.
