D-Link DI-713P - Manual (Page 7)

7
Introduction to Firewalls
A firewall is a device that sits between your computer and the Internet
that prevents unauthorized access to or from your network. A firewall can
be a computer using firewall software or a special piece of hardware built
specifically to act as a firewall. In most circumstances, a firewall is used to
prevent unauthorized Internet users from accessing private networks such
as corporate LAN’s and Intranets.
A firewall watches all of the information moving to and from your
network and analyzes each piece of data. Each piece of data is checked
against a set of criteria that the administrator configures. If any data does
not meet the criteria, that data is blocked and discarded. If the data meets
the criteria, the data is passed through. This method is called packet
filtering.
A firewall can also run specific security functions based on the type of
application or type of port that is being used. For example, a firewall can
be configured to work with an FTP or Telnet server. Or a firewall can be
configured to work with specific UDP or TCP ports to allow certain
applications or games to work properly over the Internet.