Discrete-event packet-level network simulation is well-known and widely used. Network emulation is a hybrid approach that combines real elements of a deployed networked application-such as end hosts and protocol implementations-with synthetic, simulated, or abstracted elements-such as the network links, intermediate nodes and background traffic. A key difference between the two approaches is that in the former, the notion of time is virtual and is independent of real time, whereas the latter must execute in real time. Emulation gains realism while naturally foregoing complete repeatability; historically, emulation was also tedious to control and manage. We define integrated network experimentation as spatially combining real elements with simulated elements in the same experimental run, each modeling different portions of a network topology. Integrated experiments enable new validation techniques and larger experiments than obtainable by using real elements alone. This paper highlights the key issues in integrated network experimentation, and presents some of the design techniques we use in designing, building, and putting into public production use such an integrated environment, running on a space-shared cluster.