* A firewall is a hardware or software device which is configured to permit, deny, or proxy data through a computer network which has different levels of trust.

** Function

- A firewall's basic task is to transfer traffic between computer networks of different trust levels.

- A firewall's function within a network is similar to firewalls with fire door in building construction.
In former case, it is used to prevent network intrusion to the private network. In latter case, it is intended to contain and delay structural fire from spreading to adjacent structures.
An analogy of network firewall is a fire-resistance rated wall with a fire-resistance rated, self-closing, solid-core, inside unlockable, outside key-lockable door between a house and its attached garage.

Without proper configuration, a firewall can often become worthless.

*** History
- First generation - packet filters
- Second generation - "stateful" filters
- Third generation - application layer
- Subsequent developments

****Types

There are several classifications of firewalls depending on where the communication is taking place, where the communication is intercepted and the state that is being traced.

- Network layer and packet filters
- Application-layer
- Proxies
- Network address translation