ZyXEL Communications Prestige 661H Series, User Manual

Page: 130 / 547

Prestige 661H/HW Series User’s Guide
Chapter 11 Firewalls 130
C
H A P T E R
11
Firewalls
This chapter gives some background information on firewalls and introduces the Prestige
firewall.
11.1 Firewall Overview
Originally, the term firewall referred to a construction technique designed to prevent the
spread of fire from one room to another. The networking term “firewall” is a system or group
of systems that enforces an access-control policy between two networks. It may also be
defined as a mechanism used to protect a trusted network from an untrusted network. Of
course, firewalls cannot solve every security problem. A firewall is
one of the mechanisms
used to establish a network security perimeter in support of a network security policy. It
should never be the
only mechanism or method employed. For a firewall to guard effectively,
you must design and deploy it appropriately. This requires integrating the firewall into a broad
information-security policy. In addition, specific policies must be implemented within the
firewall itself.
11.2 Types of Firewalls
There are three main types of firewalls:
• Packet Filtering Firewalls
• Application-level Firewalls
• Stateful Inspection Firewalls
11.2.1 Packet Filtering Firewalls
Packet filtering firewalls restrict access based on the source/destination computer network
address of a packet and the type of application.
11.2.2 Application-level Firewalls
Application-level firewalls restrict access by serving as proxies for external servers. Since they
use programs written for specific Internet services, such as HTTP, FTP and telnet, they can
evaluate network packets for valid application-specific data. Application-level gateways have
a number of general advantages over the default mode of permitting application traffic directly
to internal hosts: