A firewall is a collection of hardware and software that interconnects two or more networks and, at the same time, provides a central location for managing security (Morreale and Terplan 2001).
Installing a firewall between an Intranet and the Internet is a way to prevent the external security attacks. Many companies provide their employees with access to the Internet long before they give them access to an Intranet. Besides protecting an Intranet from Internet users, the company may also need to protect or isolate various departments within the Intranet from one another, particularly when sensitive information is being accessed via the Intranet. A firewall can protect the organization from both internal and external security threats (Morreale and Terplan 2001).
Most firewalls function through packet filtering based on port or address. As example: in Linux, a firewall is implemented as a series of packet-filtering rules defined by options on the iptables command line. iptables is executed once for each individual rule (Suehring and Ziegler 2006). (Different firewalls can range from a dozen rules to hundreds.)
We can classify firewalls according to its level to:
- Network level
A network-level firewall is typically a router or special computer that examines packet addresses, and then decides whether to pass the packet through or to block it from entering the Intranet.
- Application level
An application-level firewall is normally a host computer running software known as a proxy server. A proxy server is an application that controls the traffic between two networks. When using an application level firewall, the Intranet and the Internet are not physically connected.
