Definition of Internet Firewall

A firewall is a hardware device or a software program running on the secure host computer that sits between the two entities and controls access between them.

Here the two entities are nothing but a private network and the public network like Internet.

The firewall can be a software firewall and the hardware firewall.The first computer firewall was a non-routing Unix host with connections to two different networks. One network card connected to the Internet and the other to the private LAN. To reach the Internet from the private network, you had to logon to the firewall (Unix) server. You then used the resources of the system to access the Internet. For example, you could use X-windows to run Netscape's browser on the firewall system and have the display on your workstation. With the browser, running on the firewall it has access to both networks.

This sort of dual homed system (a system with two network connections) is great if you can TRUST ALL of your users. You can simple setup a Linux system and give an account accounts on it to everyone needing Internet access. With this setup, the only computer on your private network that knows anything about the outside world is the firewall. No one can download to his or her personal workstations. They must first download a file to the firewall and then download the file from the firewall to their workstation.

Firewalls are mainly used for two purposes.

1.       To keep people (worms/crackers) out.

2.       To keep people (employees/children) in.