Specifications

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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 applications. 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 pro-
tecting an Internet connection from external attacks. Most
firewalls 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 (ma-
licious applications opening backdoors on a computer)
that may have infected your computer from receiving data
on ports it created as means of accessing your computer
for potentially damaging activities. A firewall blocks most
atypical port numbers not required for normal operation of
an application and offers specialized users the opportuni-
ty to forward specified ports.
port forwarding With port forwarding it is possible to specify ports that will
allow all incoming or outgoing data packets to pass
through a router or firewall.
If a computer from the local network offers server services,
for instance, the settings of a router using NAT or IP mas-
querading must forward the port used by the server ser-
vice for access to incoming data packets and thus keep it
open permanently. The private IP address of the given
computer must be saved as the destination address for all
of the packets arriving at the port.
Typical server applications which require port forwarding
are FTP and Web servers. To allow access to a computer via
remote management software like Symantec pcAnywhere
or Microsoft’s Remote Desktop, or even use of a file-shar-
ing program like eMule, the required ports must be re-
leased for port forwarding. Port forwarding settings for the
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