1. Method for selection of a splitting point value for use in preparation of a decision tree on the basis of a set of packet processing rules for processing data packets, comprising at least the steps of selecting a splitting point candidate value from a set of rule parameter range end values in the parameter dimension being studied, changing a first counter for each rule with a first range end value being equal to said selected candidate value, changing a second counter for each rule with a second range end value being equal to said selected candidate value, and computing a goodness value for said selected candidate value at least partially on the basis of the values of said first and second counters.