The increasing number of applications offering their services over Peer-to-Peer (P2P) platforms is changing the properties of the traffic within computer networks. Their massive use raises a few imperative challenges for network administrators and Internet Service Providers, regarging the Quality of Service and security of their networks. It such scenario, it is important to develop mechanisms to control and efficiently manage the P2P traffic and prepare the networks to support it, for which it is necessary to study the effect of P2P applications in the traffic of computer networks and to develop methodologies to characterise its behaviour. In this paper, the characteristics of the traffic generated by P2P applications are analysed from the behavioural point of view, and entropy is used to measure the heterogeneity embedded in the packet sizes. The results obtained show evident difference between P2P and non-P2P traffic, being the proposed approach applicable to real-time and high-speed networks with encrypted P2P traffic, where the existing methodologies are useless.