A firewall is a program that controls what information passes from one network to
another. You can use a firewall between your PC and the Internet to stop outsiders
from getting access to your PC via the Internet.
How Firewalls Work
Each packet on the Internet is addressed to a specific port number and you can control access
by port. As a general rule, most people use only a few ports for Internet communication:
port 21 (for FTP, or file transfer), port 25 (for outgoing e-mail), port 80 (for web pages),
and port 110 (for incoming e-mail). If your computer is on a local area network, it may
use port 139 for file and printer sharing. You do not want outsiders to be able to use
this port, so you may want to block anyone on the Internet from accessing port 139.
A firewall controls which ports are open, refusing to respond to packets addressed
to other ports. Some firewalls enable you to specify what types of packets can cross the
firewall—for example, requests for web pages might be allowed outgoing, but not
incoming.
Some firewalls control only incoming information. For example, Windows ME and
XP come with the Internet Connection Firewall (ICF). ICF
prevents some types of incoming Internet traffic based on the port number, so that
hackers can’t detect that your computer is there, much less access its files.
However, with the advent of Trojan horse programs, you also need to worry about
outgoing Internet traffic. A Trojan horse is a program that installs itself on your
computer (usually arriving as a virus). It then sends packets out from your computer,
with information about your files or what you have been typing lately (including
passwords). A Trojan horse program can allow hackers to log onto your computer and
run programs on it, send e-mail (usually spam) through your computer, or cause your computer to participate in a denial-of-service attack, in which many computers
simultaneously bombard an Internet server with thousands of requests for information,
overloading the server.
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