Embodiments are directed to monitoring communication over a network using a network monitoring device (NMD) to discover devices, roles, applications, and application dependencies present on the monitored networks. A NMD may monitor network packets that may be flowing on monitored networks. Using OSI L2-to-L3 data the NMD may determine the devices that may be on the monitored networks. Also, the NMD may determine the network protocols that may be in use on the monitored networks. Further, the NMD may reassemble monitored network packets into transactions based on knowledge regarding the network protocols are in use on the monitored networks. The NMD may perform various tests to determine the applications that may be running on the discovered devices. Some of the tests used by the NMD may examine OSI L4-L7 data that may be included in the transactions.