The term firewall is derived from an automobile division. The firewall exists in the zone between the car's engine and the passenger's cabin of an automobile, and is used to prevent any heat/combustion from the engine reaching the people in the passengers cabin; in other words, it's a protective barrier. But in a network world, it is being used as a metaphorical term for how we separate our internal network from outside danger. Firewalls can be defined as any hardware or software that enables the filtering of the packets or controls the flow of traffic. They are generally implemented in a network perimeter. They act as a border for trusted and untrusted zones:

For a company, securing the network and data adds complexity. The costs of maintaining and implementing such high-level security for securing things such as e-commerce, intranet, extranet, or email services are always high, but when compared to the loss that incurred due to a lack of high-level security, it is something that is considered more important.

But if a company opts for Cisco IOS Firewall, software, instead of hardware, would also have the same kind of security satisfaction. Cisco IOS provides full-featured firewall services when it is implanted properly on any Cisco router. It helps a network to break down into several small domains or sub-networks, thereby helping by keeping the possible security breach limited to one domain, if any, and not allowing it to spread to the entire network—that would result in a major loss.

Two crucial apparatuses are used to carry out the functions of the firewall:

• An apparatus to block the traffic
• An apparatus to permit the traffic

Most firewalls would permit traffic from a trusted zone to an untrusted zone without any special configuration. But the reverse has to be configured and must be explicitly permitted, hence anything not configured/explicitly permitted from an untrusted zone to a trusted zone should be implicitly denied. A firewall is not limited to trusted and untrusted zones; there are mid-zones, generally known as DMZs (Demilitarized Zones, or less trusted zones).

Basically, a firewall is a set of programs that can be enabled in a network gateway server that secure the resources of a private network from other external network users.

The following topics will be covered:

• What is a Firewall
• Types of Firewalls