Abstract. Anomaly detection is based on profiles that represent normal behaviour of users, hosts or networks and detects attacks as significant deviations from these profiles. In the paper we propose a methodology based on the application of several data mining methods for the construction of the “normal” model of the ingoing traffic of a department-level network. The methodology returns a daily model of the network traffic as a result of four main steps: first, daily network connections are reconstructed from TCP/IP packet headers passing through the firewall and represented by means of feature vectors; second, network connections are grouped by applying a clustering method; third, clusters are described as sets of rules generated by a supervised inductive learning algorithm; fourth, rules are transformed into symbolic objects and similarities between symbolic objects are computed for each couple of days. The result is a longitudinal model of the similarity of network connections ...