1. A method for filtering packets in a real time stream of incoming: packets, comprising the steps of: creating a key from a field in a packet, the field being specified by a set of packet classification rules; transforming the key into the spectral domain, thereby obtaining a spectral domain key; comparing the spectral domain key to a spectral envelope corresponding to the classification rules and obtaining a negative decision whenever the spectral domain key falls outside the spectral envelope, or a positive decision whenever the spectral domain key falls within the spectral envelope; and in the case of a positive decision, directing the packet to a packet filter for determining the specific rule of the set of rules that the packet matches.