Denial Of Service; Basics; Figure 69 Zywall Firewall Application; Table 54 Common Ip Ports - ZyXEL Communications ZyWall 35 User Manual

Figure 69 ZyWALL Firewall Application

9.4 Denial of Service

Denials of Service (DoS) attacks are aimed at devices and networks with a connection to the
Internet. Their goal is not to steal information, but to disable a device or network so users no
longer have access to network resources. The ZyWALL is pre-configured to automatically
detect and thwart all known DoS attacks.

9.4.1 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:

Table 54 Common IP Ports

21 FTP
23 Telnet
25 SMTP
Chapter 9 Firewalls
53 DNS
80 HTTP
110 POP3
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