This article is about the network security device. For other uses, see Firewall (disambiguation).

A firewall is a dedicated appliance, or software running on another computer, which inspects network traffic passing through it, and denies or permits passage based on a set of rules.

1 Function
2 History
2.1 First generation - packet filters
2.2 Second generation - "stateful" filters
2.3 Third generation - application layer
2.4 Subsequent developments
3 Types
3.1 Network layer and packet filters
3.2 Application-layer
3.3 Proxies
3.4 Network address translation
4 See also
5 External links