A system and method for classifying packets according to packet header field values. Each of a set of subkey tables is searched for a respective packet header field value; each such search results in a value for a subkey. The subkeys are combined to form a decision key. A decision table is then searched for the decision key. The search of the decision table results in an action code and a reason code, one or both of which may be used to determine how to further process the packet.