Abstract: A method and mechanism of enforcing, in a computer network, a community separation policy wherein the data of a particular user community should be accessible only by members of that community. A Multi-Community Node (MCN) processes information for users in multiple communities and must enforce the community separation policy. In an open MCN, which may run both trusted and untrusted applications, the method and mechanism performs a set of checks on packets received from and to be transmitted on a network, and on application processes which correspond to those packets, to ensure that all communications comply with the community separation policy. The enforcement method and mechanism use a database of associations of sets of communities corresponding to ports, applications, and other network addresses within the computer network.