1. A method comprising: maintaining, by a network access device, a hierarchical key combination traffic analysis (HKCTA) table for each of a plurality of client devices to store network traffic statistics of each of the plurality of client devices, wherein the HKCTA includes a plurality of entries comprising first fields defining key identifiers for a plurality of keys and second fields comprising traffic attributes extracted from packet attributes which are to be combined in forming respective ones of the plurality of keys, each key being formed based on a combination of traffic attributes dynamically determined from packet attributes associated with a packet exchanged by a respective client device, wherein each key is formed based on a unique combination of packet attributes having at least one non-overlapped packet attribute compared to a remainder of the plurality of keys, wherein each key represents one of a plurality of hierarchical levels of a hierarchy of the plurality of entries, wherein each level of the hierarchy includes packet attributes of its parent hierarchical level plus at least one additional packet attribute that is not present in its parent hierarchical level, and wherein each hierarchical level excludes at least one packet attribute of its child hierarchical level when there is any child hierarchical level; and in response to a first packet exchanged by a first of the client devices, inspecting, by a packet inspector of the network access device, a packet attribute associated with the first packet; identifying a plurality of entries of a first HKCTA table associated with the first of the client devices, wherein the entries of the first HKCTA table are identified based on a plurality of first keys representing combinations of traffic attributes from packet attributes associated with the first packet; populating, by a traffic analysis (TA) module of the network access device, the plurality of entries of the first HKCTA table with traffic attributes extracted from network traffic data obtained from the packet attributes associated with the first packet to yield a populated HKCTA table, the network traffic data including an amount of traffic carried by the first packet transmitted or received by the first of the client devices, wherein entries in the populated HKCTA table represent network traffic statistics.