Abstract: Data elements stored in a distributed data structure are accessible by means of a hierarchical routing network in which routes through the network to individual data elements are flagged. The network includes communicating links between nodes extending from a "root node" to a plurality of end nodes. The end nodes contain the data elements. To find a data element, a search message entering the network at an end node passes through the network towards the root node until it encounters a flagged route to the relevant data element. Thereafter it passes along the route to the end node containing the relevant data element. The invention is relevant to personal numbering services in a communications network. In this case, the data elements each comprise hardware addresses for users of the network. If a user moves in relation to the network, their hardware address will change and, in many cases, the relevant end node will also change.