The invention relates to data packet filtering and finding a rule matching a data packet in a rule base. A data packet comprises parameter fields for identifying the data packet, the rule base comprises a plurality of rules, each rule comprises one or more parameter fields, and the matching rule is a rule, whose parameter field values correspond to the parameter field values of said data packet. The matching rule is found by determining rule sets for the data packet, one rule set comprising the rules to which one parameter field value of the data packet can match, and by finding the rule with the smallest label that is present in all said rule sets of the data packet, said rule with the smallest label indicating the rule matching the data packet. Additionally, the invention relates to finding an element with the smallest label that is present in a plurality of finite subsets containing finite number of elements, said subsets being subsets of a set containing finite number of sequentially labelled elements.