P-870HW-51a v2 User’s Guide
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CHAPTER 9
IP Filter
9.1 Overview
This chapter shows you how to enable and configure the ZyXEL Device IP filtering settings.
The ZyXEL Device firewall is a packet filtering firewall and restricts access based on the
source/destination computer network address of a packet and the type of application.
9.1.1 What You Can Do in this Chapter
The IP Filtering Incoming screen lets you view and configure incoming IP filtering rules
(Section 9.3 on page 108).
9.2 What You Need to Know
The following terms and concepts may help as you read through this chapter.
Basics
Computers share information over the Internet using a common language called TCP/IP. TCP/
IP, in turn, is a set of application protocols that perform specific functions. An “extension
number”, called the "TCP port" or "UDP port" identifies these protocols, such as HTTP (Web),
FTP (File Transfer Protocol), POP3 (E-mail), etc. For example, Web traffic by default uses
TCP port 80.
When computers communicate on the Internet, they are using the client/server model, where
the server "listens" on a specific TCP/UDP port for information requests from remote client
computers on the network. For example, a Web server typically listens on port 80. Please note
that while a computer may be intended for use over a single port, such as Web on port 80,
other ports are also active. If the person configuring or managing the computer is not careful, a
hacker could attack it over an unprotected port.
Some of the most common IP ports are:
Default Filtering Policies
Filtering rules are grouped based on the direction of travel of packets to which they apply.
Table 32 Common IP Ports
21 FTP 53 DNS
23 Telnet 80 HTTP
25 SMTP 110 POP3
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