What is Firewall ? Explained with Examples

By | 9th November 2015

In networking, the term firewall means a system that enforces an access control policy between networks. This control policy can include options such as a packet filtering router, a switch with VLANs, and multiple hosts with firewall software.

 A firewall system can be a composition of many different devices and components. One crucial component of a firewall is traffic filtering, which is what is mostly referred to as a firewall.

 A firewall could be likened to the metal sheet that separates the engine compartment of a vehicle or aircraft from the passenger area. Basically, the term firewall was adapted for use with computer networks; firewall is applied or configured on a network to prevent uninvited traffic from entering or gaining access to prescribed areas within a network.

The original firewalls were not standalone devices, but routers or servers with software features added to provide firewall functionality. Over time, several companies developed standalone firewalls. Dedicated firewall devices enabled routers and switches to offload the memory- and processor-intensive activity of filtering packets. Modern routers, such as the Cisco Intergrated Service Routers(ISRs), also can be used as sophisticated stateful firewalls for organizations that may not require a dedicated firewall.

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Features of Firewalls

Firewalls share some common properties:

i.  Resistant to attacks

ii. Only transit point between networks. (all traffic flows through the firewall)

iii. Enforces the access control policy

How Firewall Works

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Types of Firewalls.

Stateless Firewall.

The early firewalls were created to inspect packets to verify if they matched sets of rules, with the option of forwarding or dropping the packets accordingly. This type of packet filtering is known as stateless filtering, each packet is filtered based solely on the values of certain parameters in the packet header, similar to how ACLs (access control lists) filter packets.

Stateful Firewall.

The first stateful firewall appeared in 1989, it was developed by AT&T Bell Laboratories. This type of firewalls filter packets on information stored in the firewall based on data flowing through the firewall. The stateful firewall is able to determine if a packet belongs to an existing flow of data. They help to mitigate DoS attacks that exploit active connections through a networking device. Stateful filtering provides dynamic packet filtering capabilities to firewalls. It operates at the Network Layer of the OSI, although for some applications it can also analyze traffic at Layer 4 and Layer 5.

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Packet-filtering Firewall.

This can be in a form of a router with the capacity to filter some packet content, such as Layer 3 and sometimes Layer 4 information.They permit and deny based on Layer 4 information such as protocol, and source and destination port numbers.  Packet filtering firewall uses access control lists (ACLs) to determine whether to permit or deny traffic, based on source and destination IP addresses, protocol,source and destination port numbers, and packet type. Packet-filtering firewalls are usually part of a router firewall.

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Application Gateway Firewall or Proxy Firewall.

 A type of firewall that filters information at Layers 3, 4, 5, and 7 of the OSI reference model. Most of the firewall control and filtering is done in software.

Address-translation firewall.

 A type of firewall that expands the number of IP addresses available and conceals network addressing design.

Host-based firewall.

 A PC or server with firewall software running on it.

Transparent firewall.

 A firewall that filters IP traffic between apair of bridged interfaces.

Hybrid firewall

 A firewall that is a combination of the various firewalls types. For example, an application inspection firewall combines a stateful firewall with an application gateway firewall.

Network Security

Network Troubleshooting

Network Monitoring

Access Control Lists Explained

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