extraordinary growth of the Web is the ease with
which it allows access to information. A Web browser is a
client-server application, which means that it requires both a
client and a server component in order to function. A Web
browser presents data in multimedia formats on Web pages that
use text, graphics, sound, and video. The Web pages are created
with a format language called Hypertext Markup Language (HTML).
HTML directs a Web browser on a particular Web page to produce
the appearance of the page in a specific manner. In addition,
HTML specifies locations for the placement of text, files, and
objects that are to be transferred from the Web server to the
Web browser. Hyperlinks make the World Wide Web easy to
navigate. A hyperlink is an object, word, phrase, or picture,
on a Web page. When that hyperlink is clicked, it directs the
browser to a new Web page. The Web page contains, often hidden
within its HTML description, an address location known as a
Uniform Resource Locator (URL). In the URL
http://www.cisco.com/edu/, the "http://" tells the
browser which protocol to use. The second part,
"www", is the hostname or name of a specific machine
with a specific IP address. The last part, /education
identifies the specific folder location on the server that
contains the default web page. A Web browser usually opens to a
starting or "home" page. The URL of the home page has
already been stored in the configuration area of the Web
browser and can be changed at any time. From the starting page,
click on one of the Web page hyperlinks, or type a URL in the
address bar of the browser. The Web browser examines the
protocol to determine if it needs to open another program, and
then determines the IP address of the Web server using DNS.
Then the transport layer, network layer, data link layer, and
physical layer work together to initiate a session with the Web
server. The data that is transferred to the HTTP server
contains the folder name of the Web page location. The data can
also contain a specific file name for an HTML page. If no name
is given, then the default name as specified in the
configuration on the server is used. The server responds to the
request by sending to the Web client all of the text, audio,
video, and graphic files specified in the HTML instructions.
The client browser reassembles all the files to create a view
of the Web page, and then terminates the session. If another
page that is located on the same or a different server is
clicked, the whole process begins again. Lab Activity
Lab Exercise: Protocol Inspector, TCP and HTTP This lab is to
use Protocol Inspector, or equivalent software, to view dynamic
Transmission Control Protocol (TCP) operations. The operation
that will be specifically looked at is HTTP during web page
access. Web Links Hypertext Transfer Protocol
http://searchnetworking.techtarget.com/
search/1,293876,sid7,00.html? query=Hypertext+Transfer+Protocol&ctype= ALL
Content 11.2 The Application
Layer 11.2.5 SMTP Email servers communicate
with each other using the Simple Mail Transfer Protocol (SMTP)
to send and receive mail. The SMTP protocol transports email
messages in ASCII format using TCP. When a mail server receives
a message destined for a local client, it stores that message
and waits for the client to collect the mail. There are several
ways for mail clients to collect their mail. They can use
programs that access the mail server files directly or collect
their mail using one of many network protocols. The most
popular mail client protocols are POP3 and IMAP4, which both
use TCP to transport data. Even though mail clients use these
special protocols to collect mail, they almost always use SMTP
to send mail. Since two different protocols, and possibly two
different servers, are used to send and receive mail, it is
possible that mail clients can perform one task and not the
other. Therefore, it is usually a good idea to troubleshoot
e-mail sending problems separately from e-mail receiving
problems. When checking the configuration of a mail client,
verify that the SMTP and POP or IMAP settings are correctly
configured. A good way to test if a mail server is reachable is
to Telnet to the SMTP port (25) or to the POP3 port (110). The
following command format is used at the Windows command line to
test the ability to reach the SMTP service on the mail server
at IP address 192.168.10.5: C:\>telnet 192.168.10.5 25 The
SMTP protocol does not offer much in the way of security and
does not require any authentication. Administrators often do
not allow hosts that are not part of their network to use their
SMTP server to send or relay mail. This is to prevent
unauthorized users from using their servers as mail relays.
Web Links SMTP http://searchwebservices.techtarget.com/
sDefinition/0,,sid26_ gci214219,00.html
Content
11.2 The Application Layer
11.2.6 SNMP The Simple Network Management Protocol
(SNMP) is an application layer protocol that facilitates
the exchange of management information between network devices.
SNMP enables network administrators to manage network
performance, find and solve network problems, and plan for
network growth. SNMP uses UDP as its transport layer protocol.
An SNMP managed network consists of the following three key
components: - Network management system (NMS) –
NMS executes applications that monitor and control managed
devices. The bulk of the processing and memory resources
required for network management are provided by NMS. One or
more NMSs must exist on any managed network.
- Managed devices – Managed devices are network nodes
that contain an SNMP agent and that reside on a managed
network. Managed devices collect and store management
information and make this information available to NMSs using
SNMP. Managed devices, sometimes called network elements, can
be routers, access servers, switches, and bridges, hubs,
computer hosts, or printers.
- Agents – Agents
are network-management software modules that reside in managed
devices. An agent has local knowledge of management information
and translates that information into a form compatible with
SNMP.
Web Links Simple Network Management
Protocol http://searchnetworking.techtarget.com/ sDefinition/
0,,sid7_ gci214221,00.html
Content
11.2 The Application Layer
11.2.7 Telnet Telnet client software provides the
ability to login to a remote Internet host that is running a
Telnet server application and then to execute commands from the
command line. A Telnet client is referred to as a local host.
Telnet server, which uses special software called a daemon, is
referred to as a remote host. To make a connection from a
Telnet client, the connection option must be selected. A dialog
box typically prompts for a host name and terminal type. The
host name is the IP address or DNS name of the remote computer.
The terminal type describes the type of terminal emulation that
the Telnet client should perform. The Telnet operation uses
none of the processing power from the transmitting computer.
Instead, it transmits the keystrokes to the remote host and
sends the resulting screen output back to the local monitor.
All processing and storage take place on the remote computer.
Telnet works at the application layer of the TCP/IP model.
Therefore, Telnet works at the top three layers of the OSI