Mechanisms are provided for automatic address range detection for an IP network. Flow data is obtained comprising the source or destination IP addresses for the flow and one of: the other of the source or destination IP addresses; or direction data identifying the flow direction across the network boundary. A tree is generated representing IP addresses in the flow data. IP addresses with initial portions in common are represented in the tree with a node in common. Weights are assigned to nodes in the tree based on occurrences of the represented IP addresses in the flow data. The IP address range of the network is detected by identifying, based 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 differentiate IP addresses inside and outside the network.