ZyXEL Communications ZyXEL ZyWALL 5 - Manual (Page 156)

ZyWALL 5 User’s Guide
Chapter 9 Firewalls 155
Figure 58 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 44 Common IP Ports
21 FTP 53 DNS
23 Telnet 80 HTTP
25 SMTP 110 POP3