Network firewalls get the name from physical firewalls which prevent fire spreading from one section of a building to another. Firewalls are structures which strictly control access from one side to another.
That's exactly what a network firewall does — restrict the flow of data and program instructions in or out of a system.
There are two basic types of firewalls — hardware and software, though the distinction can be misleading. Hardware rarely does much without extra software to guide it, and software can't even exist without hardware. It has to be stored somewhere and execute on something.
Hardware firewalls are commonly used between networks — for example between a corporate network and the Internet. The Internet is nothing more than a large set of mutually cooperating networks. 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 access each other and the Internet.
Internal firewalls within corporate networks are also used, typically to keep resources restricted to certain groups, or to prevent viruses infecting some groups from spreading to others.
Software firewalls — called 'personal firewalls' — are more often used in personal computers which connect to the Internet. They're slower than hardware firewalls, which are specialized for that function, and they use resources on the individual's 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.
Browsers, e-mail programs and some auto-updating programs (such as antivirus software) all require network access. A personal firewall can be set up to allow this kind of traffic, while disallowing others.
Firewalls 'learn' which programs can access the Internet freely. The first time a program tries to connect to the Internet the firewall signals an alert. The user modifies the firewall policy by (dis)allowing that particular program to access the Internet. The rule of thumb is: if you don't recognize the program, don't allow access. Most installation software 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 can close all unused ports.
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'.
That doesn't mean, unfortunately, that personal firewalls are invulnerable. Unlike hardware firewalls, personal firewalls are part of the computer system — they're just another program — and are just as vulnerable to virus attacks as any other. Some viruses target firewalls by modifying the policy file. If this happens your computer may become vulnerable to hackers even when you thought it was secure.
Nevertheless, connecting to the Internet without using a firewall is ill-advised. Open ports are a vulnerability that can be easily exploited by hackers. To protect your firewall integrity be diligent about scanning incoming files for viruses, by using antivirus software and avoiding unexpected e-mail attachments.
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