This paper proposes a hybrid network system (called as “Virtually Isolated Network”) that combines an existing low bandwidth isolated network and the Internet, to implement a low cost overlay network with high bandwidth and high level security (precisely, information-theoretic security), without sacrificing security of the existing isolated network. Our approach consists of two main ideas: (1) Connect an isolated network and the Internet in a proper way using 4 physical unidirectional links (also known as “Data Diode” or “Air Gap”), so that the isolated network remains physically isolated; (2) Hide a small part of ciphertext from adversary by exploiting the property of isolated network and using a secret sharing approach.