Abstract: A method is described by which a data communication network can be tested. The network consists of a number of nodes which are connected to each other. At least one of the nodes is designated to the user as be inaccessible from a first node (A). Data are available within the network which, starting from the first node indicate the path to the node which is designated as being inaccessible. An interrogation is carried out to determine which is the nearest node, starting from the first node, on the path to the node which is designated as inaccessible. This nearest node is then directed to determine whether the interrogated nearest node is accessible. If this is the case, another interrogation is performed to determine which is the nearest node, now starting from the already reached node, on the path to the node which is designated as inaccessible. Another test is performed to ascertain if the new nearest node is accessible.