1.You enter a URL into the browser

It all starts here:

 2.The browser looks up the IP address for the domian name.

The first step in the navigation is to figure out the IP address for the visitied domian.The DNS lookup proceeds as follows:

  1)Browser cache(Every browser caches DNS records for different time)

  2)OS cache- If the browser cache does not contain the desired record,the browser makes a system call(gethostbyname in Windows).The OS has its own cache.

  3)Router cache-the request continues on to  your  Router,which typically has its own DNS cache.

  4)ISP DNS cache-the next place checked is ISP's DNS server,With a cache,natrually.

  5)Recursive search-Your ISP's DNS server begins a recursive search,from the root nameserver through the .com top-level nameserver,to yiqifa's nameserver.Normally,the DNS server will have names of the .com nameservers in cache,and so a hit to the root nameserver will not be necessary.

        One worrying thing about DNS is the entire domain like yiqifa.com seems to map to s single IP address.Fortunately,there are ways of mitigating the bottleneck:
           Round-robin DNS is a solution where the DNS lookup returns multiple IP addresses,rather than just one .
　　　  Load-balancer is the piece of hardware that listens on a particular IP address ande forwards the requests to other servers.Major sites will typically use expensive high-performance load balancers.
待续...