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A personal firewall (sometimes called a desktop firewall) is a software application used to protect a single Internet-connected computer from intruders. Personal firewall protection is especially useful for users with "always-on" connections such as DSL or cable modem. Such connections use a static IP address that makes them especially vulnerable to potential hackers. Often compared to anti-virus applications, personal firewalls work in the background at the device (link layer) level to protect the integrity of the system from malicious computer code by controlling Internet connections to and from a user's computer, filtering inbound and outbound traffic, and alerting the user to attempted intrusions.