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Glossary
Properties of IP Address Classes
Every IP address consists of two parts: the network ad-
dress and the host address. The sizes of the network ad-
dress and the host address are variable, and determined
by the first four bits (of the first byte) of the IP address.
IP
Masquerading
By means of IP masquerading a computer or LAN can be
protected from unauthorized connection request from the
Internet. Masquerading works by converting the IP ad-
dresses used in a network to one public IP address. From
the outside it appears that all requests are sent from a sin-
gle computer.
IP Network
A network in which data exchange takes place on the ba-
sis of the Internet Protocol is called an IP network.
Outside Dialing
Prefix
The outside dialing prefix is the digit dialed at an exten-
sion line to obtain an outside line. Generally this is “0”.
Port
So that a single network connection on a computer can be
used by multiple applications to exchange data with re-
mote 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 delivered via the IP Internet protocol. While the IP
address determines the destination computer quite gen-
erally, the port addresses the communication interface
provided by an application 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 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
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