Mechanisms are provided for automatic address range detection for an IP network. Flow data is obtained comprising one of the source and destination IP addresses for the flow and one of (a) the other of the source and destination IP addresses and (b) direction data indicative of the flow direction across the network boundary. A tree data structure is generated representing the IP addresses in the flow data. IP addresses with initial portions in common are represented in the tree with at least one node in common. Weights are assigned to nodes in the tree in dependence on occurrences of the represented IP addresses in at least a subset of the flow data. The IP address range of the network is then detected by identifying, in dependence on the assigned weights, the node associated with the last initial address portion common to all IP addresses in the network. A device is automatically configured with the IP address range to permit distinction between IP addresses inside and outside the network in operation of the device, e.g. for filtering or traffic classification.