Abstract. Active probing has increasingly been used to collect information about the topological and functional characteristics of the Internet. Given the need for active probing and the lack of a widely accepted mechanism to minimize the overhead of such probes, the traffic and processing overhead introduced on the routers are believed to become an important issue for network operators. In this paper, we conduct an experimental study to understand the responsiveness of routers to active probing both from a historical perspective and current practices. One main finding is that network operators are increasingly configuring their devices not to respond to active direct probes. In addition, ICMP based probes seem to elicit most responses and UDP based probes elicit the least. Key words: Internet measurement, active measurements