ZyXEL Communications ZYAIR, User Manual

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ZyAIR Wireless Gateway Series User’s Guide
Introduction to Firewalls 11-3
Figure 11-1 ZyAIR Firewall Application
11.4.1 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: