Today’s open platforms for network measurement and distributed system research, which we collectively refer to as testbedsin this article, provide opportunities for controllable experimentation and evaluations of systems at the scale of hundreds or thousands of hosts. In this article, we identify several issues with extending results from such platforms to Internet wide perspectives. Specifically,wetrytoquantifythelevelofinaccuracyandincompleteness oftestbedresultswhenappliedtothecontextofalarge-scalepeerto-peer (P2P) system. Based on our results, we emphasize the importance of measurements in the appropriate environment when evaluating Internet-scale systems. Categories andSubjectDescriptors C.2.5[CommunicationNetworks ]: LocalandWide-AreaNetworks– Internet; C.4 [Performance of Systems]: Measurement techniques GeneralTerms Experimentation, Performance, Measurements.