A virtual network building system includes a small terminal and an authentication server. The small terminal includes an identifier transmission unit automatically transmitting an identifier to the authentication server via a client terminal in a state in which a connection unit is connected to the client terminal, and is attachable to and detachable from the client terminal. The authentication server includes an authentication unit performing authentication on the basis of the identifier of the small terminal, a distribution unit distributing software for encrypting communication to the client terminal according to selected communication protocol and encryption method, a reception unit receiving information (access request information) regarding a request for access to the target apparatus, which is automatically transmitted from the distributed software, and a redirect unit making a proxy response of access of the client terminal to the target apparatus in response to the received access request information.
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