The invention relates to a system for data exchange between at least two communication networks using the IP Internet protocol, a first network having a security level N1 higher than the security network N2 of a second network, and several terminals being connected to the first and second networks, characterised in that it comprises at least the following members: means for marking the flows based on their security level, a gateway between the first network (1) and the network (2) for carrying out a protocol interruption, and means for filtering the data flows suitable for checking the marking and to give authorisation for the broadcasting of the flow based on the relevancy between the marker and the sensitivity level of the communication, and visual means on the terminal for indicating to the user whether or not he communicates on a secured media.