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Finding the Web Server Using DNS

As mentioned in Chapter 4, “Fundamentals of IPv4 Addressing and Routing,” a host can use
DNS to discover the IP address that corresponds to a particular hostname. URIs typically
list the name of the server—a name that can be used to dynamically learn the IP address
used by that same server. The web browser cannot send an IP packet to a destination name,
but it can send a packet to a destination IP address. So, before the browser can send a packet
to the web server, the browser typically needs to resolve the name inside the URI to that
name’s corresponding IP address.

To pull together several concepts, Figure 5-12 shows the DNS process as initiated by a web
browser, as well as some other related information. From a basic perspective, the user enters
the URI (in this case, http://www.cisco.com/go/learningnetwork), resolves the www.cisco.
com name into the correct IP address, and starts sending packets to the web server.

Free CISCO CCNA Routing and Switching ICND1 Study Guide
Figure 5-12 DNS Resolution and Requesting a Web Page

The steps shown in the figure are as follows:
1. The user enters the URI, http://www.cisco.com/go/learningnetwork, into the browser’s
address area.

2. The client sends a DNS request to the DNS server. Typically, the client learns the
DNS server’s IP address through DHCP. Note that the DNS request uses a UDP header,
with a destination port of the DNS well-known port of 53. (See Table 5-3, earlier
in this chapter, for a list of popular well-known ports.)

3. The DNS server sends a reply, listing IP address as www.cisco.com’s
IP address. Note also that the reply shows a destination IP address of, the
client’s IP address. It also shows a UDP header, with source port 53; the source port is
53 because the data is sourced, or sent by, the DNS server.

4. The client begins the process of establishing a new TCP connection to the web server.
Note that the destination IP address is the just-learned IP address of the web server.
The packet includes a TCP header, because HTTP uses TCP. Also note that the destination
TCP port is 80, the well-known port for HTTP. Finally, the SYN bit is shown,
as a reminder that the TCP connection establishment process begins with a TCP segment
with the SYN bit turned on (binary 1). At this point in the process, the web browser is almost finished setting up a TCP connection to the web server. The next section picks up the story at that point, examining how the web
browser then gets the files that comprise the desired web page.

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