|
|
| |
Well Known Ports and more
|
|
| |
By Erik Rozman
|
|
| |
A port is an identifier for an application (or
a session inside an application) within a computer, meaning
that when you use an application to connect to a computer and
request a service the local operating system assigns the requesting
application a random port number (client side) and then turns
to the requested services (server side) well known port. Ports
are defined between the Application (Winsock ,Socks) and Transport(TCP,
UDP) layer of TCP/IP-traditionally they are added either to
TCP or UDP depending on the Transport layer protocol you use.
When the requested service answers the answer is sent from
the server side service to the client side requesting applications
randomly assigned port number. |
|
| |
Scenario
The network user (199.203.185.1) wants to receive the web
site located on the web server(199.203.185.2) using a web
browsing application. |
|
| |
 |
|
 |
| IP:199.203.185.1 |
|
IP:199.203.185.2
Listening Port:80 |
|
|
| |
Request
The network user opens a web client application through which
he contacts the web server. When he contacts the web server
his client application is assigned a random (yet specific
for the current session) port number (3000) by the OS. The
client sends his IP address and randomly assigned Port number
to the web server. |
|
| |
|
|
| |
|
 |
|
IP:199.203.185.1
Source IP :199.203.185.1
Source port :3000(randomly assigned by OS)
Destination IP :199.203.185.2
Destination port:80 |
|
IP:199.203.185.2
Listening Port:80 |
|
|
| |
Reply
The web servers replies and sends the data to the client
at the IP address 199.203.185.1 and port number 3000(the
uniquely assigned port number by the OS). |
|
| |
|
|
| |
|
 |
|
IP:199.203.185.1
Listening Port:3000 |
|
IP:199.203.185.2
Port:80
Destination IP:199.203.185.1
Destination Port:3000 |
|
|
| |
Well known ports
As you see in order to reach a specific server on a specific
computer you have to know a computers IP and port number
(when they are joined together they are called sockets).
This can cause a problem, if you don’t know the specific
port number you can’t reach a specific services.
To clear the confusion IANA(Internet Assigned Numbers
Authority) created a list known as “Well Known Port Numbers” in
which they define specific ports for specific services. The total
number of available ports is 65535, the portion of “Well Known
Ports” is from 0-1023 all other ports can be freely used although
port numbers ranging between 1024 to 65535 are called “Registered
Ports”-even though it isn’t a mandatory registration ICANN
tries to keep a tab on the ports by requiring you to register a specific
port for public and published use.
The full listing can be found in RFC 1700. Note that
there are some internal TCP/IP mechanisms that will not need port
numbers since they are unique on an interface thus an IP address
is enough to designate it as unique.
|
|
| |
Well Known Ports-examples |
|
| |
| Port Number |
Protocol |
Application Layer |
| 20/21 |
TCP |
FTP |
| 80 |
TCP |
HTTP |
| 53 |
UDP/TCP |
DNS |
| 69 |
TFTP |
TFTP |
|
|
| |
By Erik Rozman MCT,
MCSE, MCSA, MCP+I, CCNA, CNA
|
|
|
 Special Promotions |
AVG Internet Security |
|
|
|
Save postage and get Key online
Weekend offer!
only 75 €
|
AVG Anti-Virus |
|
|
|
Save postage and get Key online
Weekend offer!
only 32 €
|
Get complete protection from the most dangerous threats on the internet - worms, viruses, trojans, spyware, and adware.
incl.
Anti-Spyware
"1 computer 1 year"
Get it for 32€ incl. VAT!
Save up to 15€!
PROMOTION
|
Consulting Articles 
