A firewall is a network security device that monitors and filters incoming and outgoing network traffic based on an organizations previously established security policies. It acts as a protective barrier between a private internal network and the public Internet, allowing non-threatening traffic in and keeping dangerous traffic out. Firewalls have been a first line of defense in network security for over 25 years. They establish a barrier between secured and controlled internal networks that can be trusted and untrusted outside networks, such as the Internet. Firewalls can be hardware, software, software-as-a-service (SaaS), public cloud, or private cloud (virtual) .

Firewalls can be classified into different types based on their functionality and deployment:

• Packet filtering firewalls: These were the first type of firewalls that were developed. They examine packets, or bytes, transferred between computers. They are still in use today.
• Proxy firewalls: These serve as the gateway from one network to another for a specific application. They can provide additional functionality such as content caching and security by preventing direct connections from outside the network.
• Stateful inspection firewalls: These allow or block traffic based on state, port, and protocol. They monitor all activity from the opening of a connection until it is closed. Filtering decisions are made based on both administrator-defined rules as well as context, which refers to using information from previous connections and packets belonging to the same connection.
• Next Generation Firewalls (NGFW): These focus on blocking malware and application-layer attacks, along with an integrated intrusion prevention system (IPS). They can react quickly and seamlessly to detect and combat attacks across the whole network.

Firewalls are essential components of network security and are used in both personal and enterprise settings. They play an important role in preventing cyber attacks, protecting sensitive data, and maintaining the privacy and security of computer systems and networks.