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Glossary
The Internet protocol allots 16 bits for the specification of the
port number. Thus a total of 65,535 different port numbers
can be specified. Ports up to port number 1,024 are reserved
for special system applications and typical Internet applica-
tions. These include, for instance, the port numbers 21 for
FTP (File Transfer Protocol), 25 for SMTP (Simple Mail Transfer
Protocol), 53 for DNS (Domain Name Service) and 80 for HTTP
(Hypertext Transfer Protocol).
Ports are primarily of interest to users for their role in protect-
ing an Internet connection from external attacks. Most fire-
walls offer the possibility of preventing data traffic on certain
ports. This means that certain port numbers can be blocked
to prevent data from being delivered to system services. This
is also a way of preventing any Trojans that may have infected
your computer from receiving data on ports it created as
means of accessing your PC for potentially damaging activi-
ties. A firewall blocks most of the atypical port numbers not
required for normal operation of an application and offers
specialized users the opportunity to forward specified ports.
port forwarding
With port forwarding it is possible to specify ports that will al-
low all incoming or outgoing data packets to pass through a
router or firewall.
If a PC from the local network offers server services, for in-
stance, the settings of a router using NAT
(see
glossary) or IP
masquerading
(see
glossary) must forward the port used by
the server service for access to incoming data packets and
thus keep it open permanently. The private IP address of the
given PC must be saved as the destination address for all of
the packets arriving at the port.
Typical server applications that require port forwarding are
FTP and Web servers. To allow access to a PC via remote man-
agement software like Symantec pcAnywhere or Microsoft's
Remote Desktop, or even use of a file-sharing program like
eDonkey, the required ports must be released for port for-
warding. Port forwarding settings for the most important ap-
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