A desktop firewall is a type of software that is designed to block unauthorized access to a computer. Firewalls may be either hardware devices or software, though most consumers tend to use the latter. A variety of firewalls are available from a number of different sources, though they all tend to serve the same basic functions. Many companies that are known for antivirus programs will also offer a full suite of security software, including a desktop firewall. Some firewalls are freeware while others are retail software, and some are even included with operating systems.
A typical desktop firewall will monitor and control both outgoing and incoming network traffic. This allows the firewall to catch and stop unauthorized access from an outside source, but it may also detect malware that has been installed on the computer. If an unfamiliar program attempts to access the Internet, it may be identified as malware and dealt with. In the absence of a firewall, such malicious software would be able to transmit and receive data without the user ever knowing.
Without a desktop firewall, a computer may be vulnerable to outside attacks. A computer that lacks a firewall may respond to an outside source that queries on common ports, where a firewall will generally protect against this type of activity by obscuring the ports. This may adversely affect certain applications, such as multiplayer online games, which the firewall may stop from connecting. Firewalls will often allow for port forwarding to allow applications like these to work, though it may also be necessary to turn the firewall off temporarily.
Firewalls can also regulate traffic on an Intranet, or a local network. Many will offer the ability to set different security levels for data transmitted on the Intranet, or to recognize certain computers as trusted. This may be useful in cases where a number of computers are networked together locally, since it is unlikely that they will pose the same level of threat as might come from an exterior source.
Installing a desktop firewall is not necessarily a cure-all fix for network security. If a computer already has malicious software installed, the malware itself may be able to make changes to the firewall settings. This can allow the malware to access the Internet, but it can also open the computer up for further external attacks. This may necessitate running a variety of anti-malware and antivirus programs on a regular basis to ensure the computer has not been compromised.