Firewall

A firewall is a device that is installed on a network to prevent unauthorized access of resources behind a protected network boundary. The job of the firewall might be to prevent inbound TCP connections or other data packets, while allowing outbound TCP connections. Firewalls may be deployed with very strict policied, such as blocking all inbound and outbound data packets except for very specific types of data or data to/from particular ports or IP addresses.

See Also: Network Address Translator (NAT)