Abstract: A data transmission network comprises a plurality of interconnected nodes and input/output units. In response to receipt of a data message for onward transmission through the network, each node accesses information which it stores relating to the then current topology of the network and transmits the message through the appropriate connecting links. Each node operates according to a predetermined algorithm to up-date its stored topology information. Each node at any given time assumes a particular state, state n. When in this state, it interrogates immediately each neighboring node with the question "Who are the neighbors of node of distance (n-1) from you?" where "distance" is defined in terms of the number of links. The addressed nodes respond either by returning the requested information in the form of a positive reply or by responding "Don't yet know".