A method and a computer system for securing communication in networks of data processing units which can be used especially for individually created security units for portable computer systems. In order to secure the communication, the data exchange between a data processing unit to be protected and the network is monitored and/or controlled by means of a computer program which is implemented in a security computer system as embedded software, the security computer system being inserted between the data processing device to be protected and the network. A computer system providing such protection is embodied as a single board computer or as a chip solution and comprises means for exchanging data with the data processing unit to be protected, means for exchanging data with the network, and means for monitoring and/or controlling the communication between the data processing unit to be protected and the network.