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A firewall is a network security device that monitors incoming and outgoing network traffic and decides whether to allow or block specific traffic based on a defined set of security rules.

Firewall (networking) - Simple English Wikipedia, the free encyclopedia

Firewalls have been a first line of defence in network security for over 25 years. They establish a barrier between secured and controlled internal networks that can be trusted and untrusted outside networks, such as the Internet. Read more here.

A firewall can be hardware, software, or both.

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  • Software-based firewalls: these are often run as additional programs on computers that are used for other things. They are often known as personal firewalls which can be updates on personal computers.
  • Hardware-based firewalls: Hardware based firewalls run on a dedicated computer (or appliance). Often, these offer a better performance than software firewalls, but they are also more expensive.

What firewalls cannot protect against

Firewalls can protect against some problems (viruses and attacks) that come from the internet. They cannot protect against viruses, that come from infected media (like an infected office document on an USB stick.