A method unblocks external computer systems for communication with secured processing computer systems in a computer network infrastructure. An instruction packet is transmitted from an external computer system to a broker computer system within the computer network infrastructure. A random port number of a network port is then generated by the broker computer system, and the random port number is transmitted to a processing computer system. The processing computer system at least temporarily keeps predetermined network ports closed so that access by the external computer system via network by the network ports is prevented, wherein the random port number and/or other information can be interchanged between the processing computer system and the broker computer system, however. Subsequently, the network port corresponding to the random port number is unblocked by the processing computer system, and a connection to the unblocked network port of the processing computer system is established by the external computer system.