Corporations worldwide work with teams of often dedicated system administrators to maintain, detect and prevent network infringements. This is a highly user-driven process that consumes hundreds (if not thousands) of man hours yearly. User reporting, the basis of most of these incident detection systems suffers from various biases and leads to below-par security measures. In the paper, we provide an approach for near real-time analysis of ongoing events on controlled networks, while requiring no end-user interaction and saving on system administrator's effort. Our proposed solution, ReasONets, a lightweight, distributed system, provides situational awareness in case of network incidents. ReasONets combines aspects of anomaly detection with Case-Based Reasoning (CBR) methodologies to reason about ongoing security events in a network, including their nature, severity and sources. We build a fully running prototype of ReasONets, to demonstrate the accuracy of the system, in doing reasoning and inference on the network status by exploiting events and network features. To the best of our knowledge, ReasONets is the first of its kind system combining detection and classification of network events with realtime reasoning while being capable of scaling up to large network sizes.