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Abstract — One of the first steps in communication between two Internet end systems is often a DNS lookup, mapping the domain name of the remote host to its IP address. This step introduces a compulsory delay that may have a very noticable effect on the overall loading of a Web page — particularly when a DNS query or response is lost. Depending upon server implementation, DNS timeouts may be quite high — five seconds or more — but the selection of timeout does not appear to be based on data taken from in-depth studies. In this paper we present a detailed investigation of the optimal timeout for DNS queries by DNS servers. To do so, we develop a method to investigate DNS response times, collect an appropriate sample of data, and propose an optimal timeout to be used by DNS server implementa-tions. We found that the default timeout can be reduced to one second, noticably increasing user-perceived application performance while creating an imperceptible increase in network traffic. I.