A system and method of classifying a set of internet protocol network data traffic based upon potential constant-datagram-rate data content such as VoIP traffic or streaming video using frequency spectrum analysis of datagram arrival times is disclosed. The network data traffic is captured with an apparatus connected in-line on the network link from which the set of network data traffic is obtained or wirelessly from a wireless network transmission. The network data traffic is presorted based upon parameters such as the media access control-layer, datagram size, IP address, port number or transport-layer protocol to limit the required analysis. A threshold function is used to detect peaks in the calculated frequency spectrum that indicate constant-datagram-rate traffic. An automated system preferably captures the network data traffic, immediately performs the frequency spectrum analysis on the captured data traffic, and stores the analysis results in memory for later access.