Operating Instructions and Installation Instructions

FRITZ!Box 3370 132
Glossary
delivered via the IP Internet protocol. While the IP address de-
termines the destination computer quite generally, the port
addresses the communication interface provided by an appli-
cation 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 computers) that may have infect-
ed your computer from receiving data on ports it created as
means of accessing your computer for potentially damaging
activities. 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 computer from the local network offers server services, for
instance, the settings of a router using NAT (see glossary) or
IP masquerading (see glossary) must enable or forward the
port used by the server service for access to incoming data
packets and thus keep it open permanently. The private IP ad-
dress of the given computer must be saved as the destination
address for all of the packets arriving at the port.