Ccna routing and Switching Complete Study Guide

TCP

UDP

Telnet 23

SNMP 161

SMTP 25

TFTP 69

HTTP 80

DNS 53

FTP 20, 21

BooTPS/DHCP 67

DNS 53

HTTPS 443

NTP 123

SSH 22

POP3 110

IMAP4 143

Notice that DNS uses both TCP and UDP. Whether it opts for one or the other depends 

on what it’s trying to do. Even though it’s not the only application that can use both proto-

cols, it’s certainly one that you should make sure to remember in your studies.

What makes TCP reliable is sequencing, acknowledgments, and flow con-

trol (windowing). UDP does not have reliability.

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108

 

Chapter 3 

■

  Introduction to TCP/IP 

Okay—I want to discuss one more item before we move down to the Internet layer—session  

multiplexing. Session multiplexing is used by both TCP and UDP and basically allows a 

single computer, with a single IP address, to have multiple sessions occurring simultane-

ously. Say you go to 

www.lammle.com

 and are browsing and then you click a link to another 

page. Doing this opens another session to your host. Now you go to 

www.lammle.com/forum

 

from another window and that site opens a window as well. Now you have three sessions 

open using one IP address because the Session layer is sorting the separate requests based 

on the Transport layer port number. This is the job of the Session layer: to keep application 

layer data separate!