BASIC TCP/IP AND UNDERSTANDING OF PORT NUMBERS

HOW PORT NUMBERS WORK?

When application communicates - it first choose TCP or UDP.

Then application generates source and destination port numbers.

When source communicates with server,it must establish a session.

lets say a web browser is open,source sends a message via HTTP using TCP port 80.

source  -------------------session------------host 

ip : 10.1.1.10                                              ip : 10.5.1.100

tcp port 80                                                  tcp port 80

note : 80 is port number assigned for HTTP.

- to see the list of well known port numbers ..go to the website http://www.iana.org

When you open a browser in your computer.

-Type cmd and open a cmd prompt.

-type netstat -f

this command shows the list of all

port numbers used ,ip address etc

for example :TCP  172.25.2.54:51895

here , 51895 is the source port number.

-type netstat -n

TCP/UDP both has  ( 0 to 655357) port numbers.

some well known port numbers are from 0- 1023 

TCP :

21- FTP 

22- SSH -sending email

23- TELNET 

25 - SMTP

53 - DNS SERVER

80- HTTP 

110- POP3 - post office protocol - to receive emails

443- HTTPS - secure protocol

UDP:

53- DNS CLIENT

69-TFTP