The Internet is based on finding servers through IP addresses. Because IP addresses are hard to remember, URLs typically contain hostnames instead, but the IP address is still necessary for the browser to make its request. That's the role of the Domain Name System (DNS). DNS maps hostnames to IP addresses, just as phonebooks map people's names to their phone numbers. When you type www.yahoo.com into your browser, a DNS resolver is contacted by the browser and returns that server's IP address.
This explanation highlights another benefit of DNS—a layer of indirection between URLs and the actual servers that host them. If a server is replaced with one that has a different IP address, DNS allows users to use the same hostname to connect to the new server. Or, as is the case with www.yahoo.com, multiple IP addresses can be associated with a hostname, providing a high degree of redundancy for a web site.
However, DNS has a cost. It typically takes 20–120 milliseconds for the browser to look up the IP address for a given hostname. The browser can't download anything from this hostname until the DNS lookup is completed. The response time depends on the DNS resolver (typically provided by your ISP), the load of requests on it, your proximity to it, and your bandwidth speed. After reviewing how DNS works from the browser's perspective, I'll describe what you can do to reduce the amount of time your pages spend doing DNS lookups.