Simply put, a network firewall restricts the flow of data in and out of a network. Some people use it to control the flow of information out. Others use it to prevent certain people from hacking into their systems and to protect themselves. It's not that hard to understand how firewalls work.
The following are some quick pointers about firewalls and how they work.
Hardware firewalls are commonly used between networks. A good example would be a corporate network and the Internet. How it works is that incoming traffic from the Internet is restricted to public access areas or to particular computers on the corporate network. Computers in the corporate network 'behind' the firewall can still access each other and the Internet.
Software firewalls are also known as personal firewalls. As its name suggests, it's often used on personal computers that are connected to the Internet. They aren't as elaborate as a hardware firewall but it's good enough for the user to restrict access into his pc.
Firewalls help prevent unauthorized access to systems by hackers or viruses. They control both inbound and outbound traffic according to a 'policy' established when the firewall is installed, or later configured. The policy determines which programs can access the Internet and what kind of incoming or outgoing traffic is allowed.
Firewalls 'learn' which programs can access the Internet freely. For example the first time your program tries to connect to the Internet the firewall signals an alert. You can then modify the firewall policy by allowing or disallowing that particular program to access the Internet. But if you don't recognize the program, it's better to not allow access. Most installation software for personal firewalls will specify what firewall settings are needed to run common programs properly.
Hackers routinely scan computer networks looking for open 'ports' (numbers used by network software to distinguish traffic) through which they can gain access to your computer. Ports are individual entry connection points between the computer and the Internet. If a hacker can find an open port, it can be used to transmit a worm carrying a payload which can give the hacker control over your computer. Firewalls are useful because it can close all unused ports and avoid all of this.
The best personal firewalls not only close ports, they hide their very existence. This means a hacker program scanning for open ports won't receive a reply to scan requests, rendering the system 'invisible'.
