Glossary
167
while generally there is only one public IP address available
for the Internet. To grant the network devices access to the In-
ternet, the router replaces the private IP address of the send-
er with its own public IP address in all outgoing data packets.
The router saves all necessary information in a table so that
incoming data packets can then be assigned to the correct
network device.
port
So that a single network connection on a computer can be
used by multiple applications to exchange data with remote
sites at the same time, a computer administers what are
known as ports for the IP-based protocols TCP and UDP. Ports
substantiate the point of access for the data packets deliv-
ered via the IP Internet protocol. While the IP address deter-
mines the destination computer quite generally, the port ad-
dresses the communication interface provided by an applica-
tion for a certain communication procedure.
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 (malicious applica-
tions opening backdoors on your computer) that may have in-
fected your computer from receiving data on ports it created
as means of accessing your computer for potentially damag-
ing activities. A firewall blocks most of the atypical port num-
bers not required for normal operation of an application and
offers specialized users the opportunity to forward specified
ports.
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