In order to provide more efficient transfer of data, computers
interact with the Internet through a variety of pathways known as "ports".
Keeping these ports or doorways on your computer closed to intruders
and to malware is an essential part of Internet security.
What are Internet ports?
Although your computer may have a single IP
address on the Internet, a variety
of functions and software are involved in an Internet connection . For
example, receiving email, sending email, viewing a Web page, using a newsgroup,
and uploading files are all different processes, with each using different
software methods. In order to carry out these various functions in a
systematic way, use is made of numbered "ports" as local addresses.
ports have no physical existence and are not to
be confused with actual things such as USB or parallel ports.) These
local addresses are used to direct the various types of Internet activity
to the appropriate software on the local computer.
Think of your computer
as an office building, with different rooms used to carry out various
functions. The usual IP address would correspond to the street address
of the building and the ports would correspond to room numbers. In
fact, the standard form of an URL (the human-friendly
equivalent of the numerical IP address) has a section for designating a
port. ( More details are in this
The port number is almost never necessary in an URL, however, since
port number 80 is assigned by convention to the Internet protocol http.
Ports are numbered
from 0 to 65536. However, port numbers are not assigned haphazardly
but conform to standards from the Internet
Assigned Numbers Authority (IANA). Ports 0-1023 (designated as "well
are assigned by IANA and are generally reserved for system processes.
For example, as mentioned above, the protocol http is assigned port
1024- 49151 are called "registered ports"; their assignments are
coordinated and approved by IANA. A list of these is published so that
conflicts in the use of ports do not arise. (Malware writers, of course,
do not observe these rules.) The remaining ports 49152- 65535 are called "dynamic" and/or "private
These are unregulated. Some common processes and their standard port assignments
are given in the table below.
A few common port assignments
||Domain name service
||Reading Web pages
For more information on ports, see the references given below.
Making ports secure
Since ports are used to exchange information between a computer and the Internet,
they are also a pathway for intruders to gain access to your computer or for
malware to use your computer for unauthorized activity on the Internet. Applications
or services monitor ( "listen" to) the port that they are assigned.
If this listening action is done without taking security steps, the port will
be open to incoming signals and may be vulnerable to intruders. This is where
a firewall comes
in. A firewall will monitor incoming signals and will block any that your
system has not specifically requested. Most software firewalls ( but not the
built-in Windows XP version) can also watch for outgoing traffic and will block
any that is not authorized.
If you wish to have your ports scanned
to see if they appear invisible to the outside world, there are a number of
websites that provide a free scan of the "well known" ports.
One that is well known is Steve Gibson's