When you type
www.yahoo.com into your browser, a DNS resolver is
contacted by the browser and returns the server's IP address. 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 complete. My understanding is
that this DNS lookup should happen even before you download the html
document (not the components in this document) you are requesting -
the browser needs to first find out where it is and then it can
download it.