As web pages become more complex, referencing resources from numerous domains, DNS lookups can become a significant bottleneck in the browsing experience. Whenever a client needs to query a DNS resolver over the network, the latency introduced can be significant, depending on the proximity and number of nameservers the resolver has to query (more than 2 is rare, but it can happen). As an example, the following screen shot shows the timings reported by the Page Speed web performance measurement tool. Each bar represents a resource referenced from the page; the black segments indicate DNS lookups. In this page, 13 lookups are made in the first 11 seconds in which the page is loaded. Although several of the lookups are done in parallel, the screen shot shows that 5 serial lookup times are required, accounting for several seconds of the total 11 seconds page load time.
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