A network analyzer includes a hardware-based accounting engine that generates accurate statistics for traffic within a computer network. As the network analyzer receives packets, the accounting engine associates the network packets with respective routing prefixes, and updates flow statistics for the routing prefixes. In this manner, the accounting engine maintains accurate flow statistics for all packets received by network analyzer. The network analyzer includes a control unit that generates prefix data to control the granularity of the traffic analysis. The control unit analyzes the flow statistics maintained by the accounting engine, and adaptively updates the set of prefixes to control the granularity of the statistics. The control unit may generate the prefix data as a forwarding tree having resolution nodes. Each node may associate a network prefix with forwarding next hop data, as well as respective analysis control data to enable or disable flow analysis for the prefix.