1. A method for classifying a data packet in a networked data processing device according to a plurality of differing sets of processing rules, the method comprising: receiving the data packet; parsing the data packet to identify a plurality of data items required for classifying said data packet; performing a preliminary test of at least one of the data items to determine whether any of said data item's values match known frequently-occurring values for that data item; if a match is obtained, selecting for the data item a rule range identifier associated with the matching frequently-occurring value; wherein each rule of each rule set 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 that rule applies; for those data items for which no match is obtained, performing item searches for respective data items in the parsed data packet corresponding to rule ranges defined in the plurality of differing sets of processing rules, wherein the step of performing the item search for said data item comprises: selecting a rule range identifier corresponding to the identified data item value from a predetermined set of rule range identifiers for that data item, said set of rule range identifiers indicating, 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; and wherein independent item searches are performed only once for all rule sets; combining the rule range identifiers to produce a search key; and performing respective final searches for the rule sets based on the range identifiers selected by the item searches and the preliminary tests, the final search for each rule set using the search key and predetermined rule data encoding the rule ranges for rules in that set, to identify a highest priority rule of any rules in the set which are applicable to the data packet.
1. A method for classifying a data packet in a networked data processing device according to a plurality of differing sets of processing rules, the method comprising: receiving the data packet; parsing the data packet to identify a plurality of data items required for classifying said data packet; performing a preliminary test of at least one of the data items to determine whether any of said data item's values match known frequently-occurring values for that data item; if a match is obtained, selecting for the data item a rule range identifier associated with the matching frequently-occurring value; wherein each rule of each rule set 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 that rule applies; for those data items for which no match is obtained, performing item searches for respective data items in the parsed data packet corresponding to rule ranges defined in the plurality of differing sets of processing rules, wherein the step of performing the item search for said data item comprises: selecting a rule range identifier corresponding to the identified data item value from a predetermined set of rule range identifiers for that data item, said set of rule range identifiers indicating, 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; and wherein independent item searches are performed only once for all rule sets; combining the rule range identifiers to produce a search key; and performing respective final searches for the rule sets based on the range identifiers selected by the item searches and the preliminary tests, the final search for each rule set using the search key and predetermined rule data encoding the rule ranges for rules in that set, to identify a highest priority rule of any rules in the set which are applicable to the data packet. 6. The method of claim 1 , wherein the rule sets correspond to respective processing applications of the processing device.