A data network is
[a] set of databases that are hosted on several computer systems interconnected with one another and to terminals and that serve some community of users. Such a network will typically have the following attributes: (1) The databases are dispersed over several machines; each one or group resides on one or more computer systems. (2) The computer systems are often, but not necessarily, physically distant from one another. (3) All the machines in the network are linked by some means so that information can be transmitted from one machine to another. (4) Each machine has software to permit exchange of information among individual systems in the network, and in turn to allow individual users of the network to query the many databases and to receive, analyze, and aggregate data from them.