A method for setting up a peer-to-peer communication link between a first communication medium (3) and a second communication medium (4) by means of a mediation server (5), wherein the first communication medium (3) comprises a multitude of communication ports, which are assigned in dependence on the destination by the first communication medium (3) for setting up communication links, and wherein a first communication port of the first communication medium (3) has been assigned for communication with the mediation server (5). The method further comprises the steps of composing port identification data of a second communication port yet to be assigned of the first communication medium (3), the first communication medium (3) sending the port identification data to the second communication medium (4) via the mediation server (5), and the first communication medium (3) assigning the second communication port for setting up the peer-to-peer communication link.