Computer and network administrators are often confused or uncertain about the behavior of their networks. Traditional analysis using IP ports, addresses, and protocols are insufficient to understand modern computer networks. Here we describe NetADHICT, a tool for better understanding the behavior of network traffic. The key innovation of NetADHICT is that it can identify and present a hierarchical decomposition of traffic that is based upon the learned structure of both packet headers and payloads. In particular, it decomposes traffic without the use of protocol dissectors or other application-specific knowledge. Through an AJAX-based web interface, NetADHICT allows administrators to see the high-level structure of network traffic, monitor how traffic within that structure changes over time, and analyze the significance of those changes. NetADHICT allows administrators to observe global patterns of behavior and then focus on the specific packets associated with that behavior, acting a...