Methods and apparatus are provided for classifying data packets in a data processing device. In one aspect, packets are classified according to a plurality of sets of processing rules. Each rule defines a plurality of rule ranges, each rule range indicating a range of possible values of a corresponding data item in the packet for which the rule applies, and rule ranges defined in different rule sets correspond to a common set of data items. Independent item searches are performed for respective data items in the packet corresponding to rule ranges defined in the rule sets. The item search for a data item comprises selecting a range identifier corresponding to the value of the data item from a predetermined set of range identifiers for that data item. This set of range identifiers indicates, for all possible values of the data item, which of the rule ranges corresponding to the data item in the rule sets a value intersects. Next, respective final searches are performed for the rule sets based on range identifiers selected by the item searches. The final search for each rule set uses the selected range identifier for each data item corresponding to a rule range defined in that rule set, and predetermined rule data encoding the rule ranges for rules in that set, to identify at least one rule of any rules in the set which are applicable to the data packet. Thus packets can be classified according to multiple rule sets, but the initial item searches are performed only once, for all the rule sets.