参考：
http://developer.yahoo.com/performance/rules.html#dns_lookups
DNS
就像电话本，根据电话本查找指定网站的
IP
地址。查找一次
DNS
一般需要
20-120
毫秒。
The Domain Name System (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 contacted by the
browser returns that server's IP address. DNS has a cost. It typically takes 20-120
milliseconds for DNS to lookup the IP address for a given hostname.
The
browser can't download anything from this hostname until the DNS lookup is
completed.
DNS缓存可以提高性能。DNS缓存可以发生在缓存服务器上，由ISP或本地局域网维持。在个人计算机的操作系统中也有DNS信息缓存。还有，大部分的浏览器都有自己的DNS缓存。
DNS lookups are cached for better performance. This caching
can occur on a special caching server, maintained by the user's ISP or local
area network, but there is also caching that occurs on the individual user's
computer. The DNS information remains in the operating system's DNS cache (the
"DNS Client service" on Microsoft Windows). Most browsers have their
own caches, separate from the operating system's cache. As long as the browser
keeps a DNS record in its own cache, it doesn't bother the operating system
with a request for the record.
IE浏览器默认保持DNS缓存30分钟，Firefox默认保持1分钟，Fasterfox保持1小时。
Internet Explorer caches DNS lookups for 30 minutes by
default, as specified by the DnsCacheTimeout
registry setting.
Firefox caches DNS lookups for 1 minute, controlled by the network.dnsCacheExpiration
configuration setting. (Fasterfox changes this to 1 hour.)
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
.
减少DNS查找可以缩短响应时间，但是减少了并发下载可能导致响应时间增加。
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. 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.