Why Everyone Is Talking About These Firewall Software - By: Karl Sultana

A firewall is dedicated software running on computer, which inspects network traffic passing through it, and denies or permits passage based on a set of rules.
A firewall's basic task is to regulate some of the flow of traffic between computer networks of different trust levels. Typical examples are the Internet which is a zone with no trust and an internal network which is a zone of higher trust. A zone with an intermediate trust level, situated between the Internet and a trusted internal network, is often referred to as a "perimeter network" or Demilitarized zone (DMZ).
A firewall's function within a network is similar to firewalls with fire doors in building construction. In the former case, it is used to prevent network intrusion to the private network. In the latter case, it is intended to contain and delay structural fire from spreading to adjacent structures.
Without proper configuration, a firewall can often become worthless. Standard security practices dictate a "default-deny" firewall rule set, in which the only network connections which are allowed are the ones that have been explicitly allowed. Unfortunately, such a configuration requires detailed understanding of the network applications and endpoints required for the organization's day-to-day operation. Many businesses lack such understanding, and therefore implement a "default-allow" rule set, in which all traffic is allowed unless it has been specifically blocked. This configuration makes inadvertent network connections and system compromise much more likely.
Types of firewalls
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
Network layer firewalls, also called packet filters, operate at a relatively low level of the TCP/IP protocol stack, not allowing packets to pass through the firewall unless they match the established rule set. The firewall administrator may define the rules; or default rules may apply. The term packet filter originated in the context of BSD operating systems.
Application-layer Firewall
Application-layer firewalls work on the application level of the TCP/IP stack i.e., all browser traffic, or all telnet or ftp traffic and may intercept all packets traveling to or from an application. They block other packets usually dropping them without acknowledgement to the sender. In principle, application firewalls can prevent all unwanted outside traffic from reaching protected machines.
ProxiesFirewall
A proxy device may act as a firewall by responding to input packets, (connection requests, for example) in the manner of an application, whilst blocking other packets. Proxies make tampering with an internal system from the external network more difficult and misuse of one internal system would not necessarily cause a security breach exploitable from outside the firewall.
Network address translation
Firewalls often have network address translation (NAT) functionality, and the hosts protected behind a firewall commonly have addresses in the "private address range", as defined in RFC 1918. Firewalls often have such functionality to hide the true address of protected hosts. Originally, the NAT function was developed to address the limited number of IPv4 routable addresses that could be used or assigned to companies or individuals as well as reduce both the amount and therefore cost of obtaining enough public addresses for every computer in an organization. Hiding the addresses of protected devices has become an increasingly important defense against network reconnaissance

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