Firewall

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A firewall is a computer, router or other communications device which filters access to a protected network.
It allows a company to protect its network from unwarranted intrusion from the Internet whilst permitting its employees to get acces to Internet services such as web and email. Firewalls can also be used as an access control measure to only allow certain people within the organisation access to the Internet. Many firewalls now contain features to control, authenticate and secure users who may want to access a company's internal data from the Internet or even another company.

Firewalls are used in many organisations, not just those with Internet connections. They can be used to protect mainframe or other central resources from general access within an organisation or to ensure confidentiality of information passing across public networks (whether internal or external).

You can find further information under:

Proxy server