Since the early 2000's, the Internet topology has been an attractive and important research topic, either for developing data collection mechanisms, and for analyzing and modeling the network. Beside traditional aspects of the Inter-net topology (i.e., IP interface, router, and AS levels), recent researches focused on intermediate promising visions of the topology, namely Point-of-Presence (PoP) and subnets (i.e., a set of devices that are located on the same connection medium and that can communicate directly with each other at the link layer). This paper focuses on network subnet discovery by proposing a new tool called TreeNET. One of the key aspects of TreeNET is that it builds a tree representing the way subnets are located with respect to each other. This tree allows TreeNET to obtain additional information on the network, leading to better analysis of the collected data. In this paper, we demonstrate the potential of TreeNET through the evaluation of its key algorithmic steps and the study of measurements collected from the PlanetLab testbed.