The phrase "closed network" has been used to mean several different things:

• A "closed network" can refer to a private telephone network that has no external (public switched telephone network) connectivity.
• A "closed network" can refer to a network that employs proprietary technology that is not directly interoperable with other standards-based networks.
• A "closed network" can refer to a private network that can only be used by authenticated, authorized devices; outsider use is prohibited and enforced through cryptographic means.
• A "closed network" can refer to a WLAN that does not send its name (SSID) in beacon frames; stations must know the SSID in order to connect to access points in that network.

You are probably looking for the first definition; an example of that kind of closed network can be seen in this...