ZyXEL Communications ZYWALL10 - Manual (Page 160)

ZyWALL 10 Internet Security Gateway
What Is a Firewall? 13-3
Figure 13-1 ZyWALL Firewall Application
13.3 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.
13.3.1 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. These protocols, such as HTTP (Web), FTP (File
Transfer Protocol), POP3 (E-mail), etc., are identified by an “extension number”, called the "TCP port" or
"UDP port". 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.