Ccna routing and Switching Complete Study Guide

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f I g u r e   3 . 2     The TCP/IP protocol suite

DoD Model

Telnet

TFTP

FTP

SMTP

LPD

NFS

SNMP

X Window

Application

Transport

ICMP

ARP

RARP

IP

Internet

Link

TCP

UDP

Ethernet

Fast

Ethernet

Token

Ring

FDDI

In the following sections, we will look at the different protocols in more detail, begin-

ning with those found at the Process/Application layer.

The Process/Application Layer Protocols

Coming up, I’ll describe the different applications and services typically used in IP net-

works, and although there are many more protocols defined here, we’ll focus in on the  

protocols most relevant to the CCNA objectives. Here’s a list of the protocols and applica-

tions we’ll cover in this section:

 

■

Telnet

 

■

SSH

 

■

FTP

 

■

TFTP

 

■

SNMP

 

■

HTTP

 

■

HTTPS

 

■

NTP

 

■

DNS

 

■

DHCP/BootP

 

■

APIPA

Telnet

Telnet was one of the first Internet standards, developed in 1969, and is the chameleon of 

protocols—its specialty is terminal emulation. It allows a user on a remote client machine, 

called the Telnet client, to access the resources of another machine, the Telnet server, in order 

to access a command-line interface. Telnet achieves this by pulling a fast one on the Telnet 

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Chapter 3 

■

  Introduction to TCP/IP 

server and making the client machine appear as though it were a terminal directly attached to 

the local network. This projection is actually a software image—a virtual terminal that can 

interact with the chosen remote host. A drawback is that there are no encryption techniques 

available within the Telnet protocol, so everything must be sent in clear text, including pass-

words! Figure 3.3 shows an example of a Telnet client trying to connect to a Telnet server.

f I g u r e   3 . 3    Telnet

>telnet 1.1.1.2

Can I have access to

your command line?

Okay! Configure me!

I’ll send everything in

clear text, including

passwords...

Good, because I can’t

do encryption!

These emulated terminals are of the text-mode type and can execute defined procedures 

such as displaying menus that give users the opportunity to choose options and access the 

applications on the duped server. Users begin a Telnet session by running the Telnet client 

software and then logging into the Telnet server. Telnet uses an 8-bit, byte-oriented data 

connection over TCP, which makes it very thorough. It’s still in use today because it is so 

simple and easy to use, with very low overhead, but again, with everything sent in clear 

text, it’s not recommended in production.