High Performance Web Sites

Reducing DNS Lookups

When the client's DNS cache is empty (for both the browser and the operating system), the number of DNS lookups is equal to the number of unique hostnames in the web page. This includes the hostnames used in the page's URL, images, script files, stylesheets, Flash objects, etc. Reducing the number of unique hostnames reduces the number of DNS lookups. Google (http://www.google.com) is the preeminent example of this, with only one DNS lookup necessary for the page.

Reducing the number of unique hostnames has the potential to reduce the amount of parallel downloading that takes place in the page. Avoiding DNS lookups cuts response times, but reducing parallel downloads may increase response times. As described in Chapter 8 in the section "Parallel Downloads," some amount of parallelization is good, even if it increases the number of hostnames. In the case of Google.com, there are only two components in the page. Because components are downloaded two per hostname in parallel, using one hostname minimizes the number of possible DNS lookups while maximizing parallel downloads.

Most pages today have a dozen or more components—not nearly as lean as Google. My guideline is to split these components across at least two but no more than four hostnames. This results in a good compromise between reducing DNS lookups and allowing a high degree of parallel downloads.

The advantage of using Keep-Alive, described in Chapter 2, is that it reuses an existing connection, thereby ...