Windows 8 Bible

How Firewalls Work

To understand what a firewall is, you need to first understand what a network connection is. Even though you have only one skinny set of wires connecting your computer to the Internet (through a DSL phone line or cable outlet), that connection actually consists of 65,535 ports. Each port can simultaneously carry on its own conversation with the outside world. So, theoretically, you could have 65,535 things going on at a time. Of course, nobody ever has that much going on all at one time. A handful of ports is more like it.

The ports are divided into two categories:

• TCP (Transmission Control Protocol): This is generally used to send text and pictures (web pages and e-mail), and includes some error checking to make sure all the information that’s received by a computer matches what the sending computer sent.
• UDP (User Datagram Protocol): This works more like broadcast TV or radio, where the information is just sent out and there is no error checking. UDP is generally used for real-time communications, such as voice conversations and radio broadcasts sent over the Internet.

Each port has two directions: incoming (or ingress) and outgoing (or egress). The direction is in relation to stuff coming into your computer from the outside: namely the Internet. It’s the stuff coming into your computer that you have to watch out for. But you can’t close all ports to all incoming traffic. If you did, there’d be no way to get the good stuff in. But you don’t want to let everything ...