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Glossary Contents

A firewall is a network security system that monitors and controls incoming and outgoing network traffic based on predetermined security rules. It establishes a barrier between a trusted internal network and an untrusted external network, such as the Internet.

The term firewall originally referred to a wall intended to confine a fire within a building. Later uses refer to similar structures, such as the metal sheet separating the engine compartment of a vehicle or aircraft from the passenger compartment. The term was applied in the late 1980s to network technology that emerged when the Internet was fairly new in terms of its global use and connectivity.

Types of Firewalls

Firewalls are categorised into two types: network firewalls and host-based firewalls. Network firewalls filter traffic between two or more networks and run on network hardware. Host-based firewalls run on host computers and control network traffic in and out of those machines.

Firewalls can be further divided into different types based on their structure and method of operation. These include Packet Filtering Firewalls, Stateful Inspection Firewalls, Proxy Firewalls, Next-Generation Firewalls, and Software and Hardware Firewalls.

Packet Filtering Firewalls

Packet filtering firewalls, the most traditional type of firewall, inspect packets (small chunks of data) and prevent them from passing through if they fail to match an established security rule set. The firewall has a list of firewall rules that can block traffic based on IP protocol, IP address and/or port number.

Packet filtering firewalls work mainly on the network layer of the OSI model but also process some upper-layer information. They are effective and transparent to users, but they are difficult to configure and susceptible to IP spoofing.

Stateful Inspection Firewalls

Stateful inspection firewalls, also known as dynamic packet filtering firewalls, keep track of active connections and use this information to determine which network packets to allow through. They offer more security than traditional packet-filtering firewalls, but they also require more processing power.

Stateful inspection firewalls monitor all aspects of a network connection and are capable of assessing the state and context of a packet in a more detailed way, which allows for a higher level of security.

Firewall Configuration

Firewall configuration involves setting up the firewall rules that dictate what traffic is allowed through and what traffic is blocked. This process is critical to the successful operation of the firewall and the security of the network it protects.

Firewall rules are based on several factors, including IP addresses, domain names, protocols, programs, ports, and keywords. These rules can be configured to block specific types of traffic, allow specific types of traffic, or trigger an alert when certain types of traffic are detected.

Rule-Based Configuration

Rule-based configuration involves setting up a list of rules that the firewall follows when inspecting network traffic. These rules can be based on any number of factors, including IP addresses, domain names, protocols, programs, ports, and keywords.

Each rule in a rule-based configuration specifies a set of conditions and an action. The conditions define the characteristics of the network traffic, and the action defines what the firewall should do when it encounters traffic that matches the conditions.

Policy-Based Configuration

Policy-based configuration involves setting up a policy that dictates how the firewall should handle different types of network traffic. Policies can be based on various factors, including the type of traffic, its source, its destination, and the time of day.

Policy-based configuration is often used in large organisations where the network traffic is complex and varied. It allows for more flexibility and control than rule-based configuration, but it can also be more difficult to set up and manage.

Firewall Limitations

While firewalls are critical components of network security, they are not perfect. Several limitations and potential issues can arise when using a firewall.

One of the main limitations of firewalls is that they cannot protect against attacks that do not go through them. For example, if a user brings a laptop to a coffee shop and connects to an insecure public Wi-Fi network, the laptop could be infected with malware that could then spread to the corporate network when the laptop is brought back to the office.

False Positives and False Negatives

False positives and false negatives are common issues with firewalls. A false positive occurs when a firewall incorrectly identifies legitimate network traffic as malicious and blocks it. This can disrupt network services and lead to lost productivity.

On the other hand, a false negative occurs when a firewall fails to identify malicious network traffic and allows it through. This can lead to security breaches and data loss. Balancing the sensitivity of the firewall to minimise both false positives and false negatives is a key challenge in firewall management.

Performance Impact

Firewalls can significantly impact network performance. The processing power required to inspect all incoming and outgoing network traffic can slow down network speeds, especially if the firewall is not properly configured or is handling a large volume of traffic.

Performance impact can be mitigated by using a high-performance firewall, by optimising the firewall's configuration, or by using a firewall with load-balancing capabilities.

Firewall Evolution

Firewalls have evolved significantly since their inception in the late 1980s. Early firewalls were simple packet filters, but today's firewalls are much more sophisticated and offer a wide range of security features.

Modern firewalls can protect against a variety of threats, including viruses, worms, bots, and denial-of-service attacks. They can also provide VPN services, intrusion detection and prevention, and advanced threat detection and response capabilities.

Next-Generation Firewalls

Next-generation firewalls (NGFWs) are a type of firewall that combines traditional firewall technology with additional functionality, such as deep packet inspection, intrusion prevention systems, and application awareness.

NGFWs provide a higher level of security than traditional firewalls by inspecting the payload of network packets and making decisions based on the content of the packets, not just the header information. They can also identify and control applications, users, and devices on the network.

Cloud-Based Firewalls

Cloud-based firewalls, also known as Firewall-as-a-Service (FaaS), are a new type of firewall that is hosted in the cloud rather than on-premise. They provide the same functionality as traditional firewalls, but they are more scalable and easier to manage.

Cloud-based firewalls can be deployed quickly and easily, without the need for any hardware or software installation. They also offer centralised management and reporting, which can simplify the task of managing multiple firewalls across a large organisation.


In conclusion, firewalls play a crucial role in network security by controlling incoming and outgoing network traffic based on predetermined security rules. They are a critical component of any organisation's security infrastructure, protecting sensitive data and systems from external threats.

While firewalls have their limitations and potential issues, the benefits they provide in terms of security and control far outweigh these challenges. As network threats continue to evolve, so too will firewalls, with new technologies and capabilities being developed to meet these challenges head-on.

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