Abstract: Methods and apparatus are provided for classifying data packets in data processing systems. A first packet classification method determines which of a plurality of predefined processing rules applies to a data packet, where each rule is associated with a range of possible data values in each of a plurality of dimensions (X,Y) corresponding to respective data items in the packet format. For each dimension (X,Y), it is determined which of a set of predefined basic ranges contains the corresponding data value (I1, I2) from the packet, where the basic ranges correspond to respective non-overlapping value ranges between successive rule range boundaries in the dimension. For the basic range so determined for each dimension, a corresponding basic range identifier is selected from a set of predefined basic range identifiers corresponding to respective basic ranges in that dimension.