Firewall: What It Is, How It Works, and Types

A firewall is a security system that monitors the flow of network traffic sent and received by a computer or network. It establishes rules for allowing authorised traffic through and blocks everything else that could cause problems. (They also protect a whole network of computers and devices). In this guide, we explain what a firewall is, what it’s used for, and how this system works. We also cover the different types available and the Windows firewall.

What Is a Firewall?

In computer science, and more specifically in telecommunications, a firewall is understood to be a system or device capable of allowing, limiting, encrypting or decoding communication traffic between a computer (or a local network) and the rest of the Internet, preventing unauthorized users or systems from gaining access.

A firewall can be either physical or digital, that is, a device or a piece of software, and in both cases it constitutes only a first line of defence in the world of computer networks, indispensable but not sufficient to guarantee security against cyberattacks and intrusions by third parties.

These types of technologies take their name from a fire prevention mechanism typically used in green areas or vulnerable buildings, which consists of a wall or a vacant area free of any flammable content, limiting the indefinite spread of fire in the event of a fire.

The precursors to firewalls emerged during the 1980s, when the Internet was still a recent field of knowledge and the potential for damage through hacking or cyberterrorism had not yet been considered.

Since the first massive cyberattacks by digital viruses and worms at the beginning of the following decade, digital protection has become increasingly sophisticated, keeping pace with the emergence of new forms of harmful software, known as malware.

What Is a Firewall Used For?

Simply put, a firewall serves to interrupt unauthorized or suspicious connections between a computer system and the internet. This cuts off remote communication between malicious users (hackers) and the local system, or between malware surreptitiously installed on the system and the outside world, thus preventing it from acting as the spearhead of a cyberattack.

It is, therefore, a monitor that prevents unauthorized or suspicious communications from entering or exiting, based on discernment criteria that it updates periodically.

How Does a Firewall Work?

Firewalls distinguish between permitted and dangerous or suspicious connections, based on different procedures, such as:

Firewall Policies

Using IP addresses and other identification systems, the firewall suspends any communication request that does not originate from the internal network or the system itself, disguising all internal resources behind a unique IP address so that no one can monitor them from the outside.

Content Filtering

Through a system of exclusion rules where the user has the final say, the firewall distinguishes between problematic, suspicious, or unsafe content and content that is left to the user’s discretion. Thus, access to websites or entire servers can be blocked as a precaution.

Antimalware Services

Many firewalls incorporate virus and malware definitions from various defensive programs, thus helping to contain the spread of these harmful programs.

Deep Packet Inspection (DPI)

This term refers to Deep Packet Inspection (DPI) procedures, which add a second layer of security to the system by reviewing the deep content of received information packets.

What Are the Different Types of Firewalls?

The following types of firewalls exist:

• Application-Level Gateway: This type applies only to specific applications considered risky, such as FTP servers or peer-to-peer (P2P) file sharing. It is usually very effective, but it does impose a reduction in system resources.
• Circuit-Level Gateway: Monitors the establishment of TCP or UDP connections through secure sessions.
• Network-Layer Firewall: It operates by inspecting IP addresses and IP packet exchanges, and it uses alternative data such as the MAC address.
• Application-Layer Firewall: It operates based on applications, controlling how they access the Internet, for example, through proxies.
• Personal Firewalls: Firewalls are chosen by the user and installed on the system to meet their individual usage requirements.

What Is Windows Firewall?

Microsoft’s renowned operating system, Windows, includes firewall software among its automatic defence options. This feature was added to the system’s Security Center starting with Windows XP Service Pack 2, aiming to provide users with greater control and stability for their system.

This firewall updates along with other Windows applications to stay current on cyber threats, and it is automatically activated unless there is other antivirus or firewall software that has connection management privileges, in which case the Windows firewall will remain disabled.

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