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 a 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; representing the relation of rules in comparison with the splitting point candidate by a first value representing the number of rules whose both range end values are below the splitting point candidate; a second value representing the number of rules whose low range end value is smaller than the splitting point candidate but whose high range end value is equal to the splitting point candidate; a third value representing the number of rules whose low range end value is equal to the splitting point candidate but whose high range end value is larger than the splitting point candidate; a fourth value representing the number of rules whose low range end value is lower than the splitting point candidate but whose high range end value is larger than the splitting point candidate; and a fifth value representing the number of rules whose both range end values are higher than the splitting point candidate; computing a goodness value for said selected candidate value at least partially on the basis of the values of said first and second counters; and storing the goodness value for said selected candidate value in a computer readable medium.