Accessing Web Sources - Beginning Java Programming: The Object-Oriented Approach - page 350

Note The excellent free and open source Wireshark tool is used to capture
and show network packets here.

Figure 10-1 shows the “stack” of protocols discussed previously. At the top, you can see
a “frame” message, which describes your message in its most basic “physical” terms—a
series of bytes, 0 and 1 pulses on a wire. Below this, you find “Ethernet,” which in this
case shows that communication is going on between a Dell laptop and a Cisco router (the
manufacturer information can be derived from a unique identifier which is associated
with every Ethernet device). Under this, you have the Internet Protocol, which shows that
a message is sent from the address 10.33.200.175 (the laptop) to 8.8.8.8 (Google's DNS
server). The IP protocol includes some basic checksums to make sure the receiver can check
the integrity of the package (errors occur when the transmission is sent over wires). Next,
you see the “User Datagram Protocol” (UDP), which shows that a message is being sent to
port 53, the standard port DNS servers are listening on. Assigning messages a destination
port number provides a simple way for the receiving party to know for which application
the message is intended. The computer also specifies a temporary source port (61796) to
indicate that a reply is expected to be sent to this port. Finally, at the bottom, you find
the “Domain Name System” (DNS) message, which tells you that this message contains a
query for www.wrox.com .

5.

You're not out of the woods yet, as the DNS server still has to send back a reply. Again, a
complex message is constructed and sent back, as shown in Figure 10-2.

figure 10-2