2.5 DNS Use Example

1.	Suppose that a web browser user is requesting the page www.somesite.com/index.html. For the user’s host to be able to send an HTTP request message to the web server www.somesite.com, the user’s host is required to determine its IP address. This is done as follows:

	a)	The browser passes the hostname www.somesite.com to its local DNS server.
	b)	The local DNS server sends a query containing the hostname to a DNS root server.
	c)	The root DNS server sends back to the local DNS server a list of IP addresses for TLD servers responsible for the .com top level domain (determined by com suffix of the hostname).
	d)	The local DNS server then resends the request to one of the TLD DNS servers.
	e)	The TLD server sends back the IP address for the authoritative DNS server for the organization running the somesite.com site.
	f)	The local DNS server then resends the message directly to the authoritative DNS server of the organization running somesite.com, which sends back the IP address of the host somesite.com.

Note that DNS caching can be used to reduce the number of DNS messages required to get the domain name-IP mapping.