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Introduction of Firewall in Computer Network

Last Updated : 12 Aug, 2025

A firewall is a network security device, either hardware or software-based, which monitors all incoming and outgoing traffic and based on a defined set of security rules, it accepts, rejects, or drops that specific traffic. It acts like a security guard that helps keep your digital world safe from unwanted visitors and potential threats.

• Accept: allow the traffic
• Reject: block the traffic but reply with an “unreachable error”
• Drop: block the traffic with no reply

A firewall is essential because networks are constantly exposed to both safe and harmful traffic from the internet or other networks. Without a firewall, your systems would have no protection against unwanted access, malicious activity, or accidental data leaks.

Imagine your house door is always open. Anyone passing by could walk in and take your things.
A firewall is like a locked door with a guard, letting only trusted people in and keeping strangers out.

Think of your email inbox. Without a spam filter, you’d get flooded with scam and spam messages. A firewall works like that spam filter it blocks harmful data before it reaches you.

It’s like keeping your bank PIN in a safe instead of leaving it on the table where anyone can see it. A firewall ensures your personal and business data stays hidden from cybercriminals.

If you leave your car unlocked in a parking lot, thieves can steal it. A firewall locks your network so attackers can’t hijack it.

Just like parents set parental controls so kids can’t visit unsafe websites, Firewalls control where your computers are allowed to connect.

Here’s a clear explanation of the working of a firewall, including its types and flow

A firewall works like a security guard for your network, standing between your internal systems such as computers, servers, and devices and the outside world, like the internet or other networks. It carefully inspects all data entering or leaving to ensure only safe traffic is allowed through.

• When data tries to enter or leave your network, it passes through the firewall first.
• The firewall examines the data packets (small chunks of information) using predefined rules.
• Rules can be defined on the firewall based on the necessity and security policies of the organization.
• FIrewall allows decision making like Allow → If the packet matches safe rules. or Block → If the packet is suspicious, from a blacklisted source, or contains malicious code.
• The firewall records blocked or unusual traffic for security teams to review.
• Alerts can be sent in real time if a major threat is detected.

Default policy: It is very difficult to explicitly cover every possible rule on the firewall. For this reason, the firewall must always have a default policy. Default policy only consists of action (accept, reject or drop). Suppose no rule is defined about SSH connection to the server on the firewall. So, it will follow the default policy. If default policy on the firewall is set to accept, then any computer outside of your office can establish an SSH connection to the server. Therefore, setting default policy as drop (or reject) is always a good practice.

Firewalls can be categorized based on their generation.

• Packet Filtering Firewall
• Stateful Inspection Firewall
• Proxy Firewall (Application Level)
• Circuit-Level Gateway
• Web Application Firewall (WAF)
• Next-Generation Firewall (NGFW)

• Network Firewall
• Host-Based Firewall

• Perimeter Firewall
• Internal Firewall
• Distributed Firewall

• Hardware Firewall
• Software Firewall

A network firewall is your first line of defense in cybersecurity. It monitors, filters, and controls data moving in and out of your network

• Networks are vulnerable to any traffic trying to access your systems, whether it's harmful or not. That's why it's crucial to check all network traffic.
• When you connect personal computers to other IT systems or the internet, it opens up many benefits like collaboration, resource sharing, and creativity. But it also exposes your network and devices to risks like hacking, identity theft, malware, and online fraud.
• Once a malicious person finds your network, they can easily access and threaten it, especially with constant internet connections.
• Using a firewall is essential for proactive protection against these risks. It helps users shield their networks from the worst dangers.

Firewalls evolved from simple packet filtering to advanced, user-friendly security systems used worldwide.

• Late 1980s: Jeff Mogul, Brian Reid, and Paul Vixie at Digital Equipment Corp (DEC) developed packet-filtering technology, laying the groundwork for firewalls by checking external connections before they reached internal networks.
• Late 1980s - Early 1990s: AT&T Bell Labs researchers, including Presotto, Sharma, and Nigam, developed the circuit-level gateway, a firewall that vetted ongoing connections without reauthorizing each data packet, paving the way for more efficient security.
• 1991-1992: Marcus Ranum introduced security proxies at DEC, leading to the creation of the Secure External Access Link (SEAL), the first commercially available application-layer firewall, based on earlier DEC work.
• 1993-1994: At Check Point, Gil Shwed pioneered stateful inspection technology, filing a patent in 1993. Nir Zuk developed a graphical interface for Firewall-1, making firewalls accessible and widely adopted by businesses and homes

A firewall serves as a security barrier for a network, narrowing the attack surface to a single point of contact. Instead of every device on a network being exposed to the internet, all traffic must first go through the firewall. This way, the firewall can filter and block non-permitted traffic, whether it's coming in or going out. Additionally, firewalls help create a record of attempted connections, improving security awareness.

Firewalls regulate both inbound and outbound traffic, protecting the network from:

• External threats such as viruses, phishing emails, denial-of-service (DoS) attacks, and backdoors. Firewalls filter incoming traffic flows, preventing unauthorized access to sensitive data and thwarting potential malware infections.
• Insider threats like known bad actors or risky applications. A firewall can enforce rules and policies to restrict certain types of outgoing traffic, which helps identify suspicious activity and mitigate data exfiltration.

Firewalls can protect against a variety of threats by monitoring and controlling incoming and outgoing network traffic. Here are the main things they help defend against:

• Infiltration by Malicious Actors: Firewalls can block suspicious connections, preventing eavesdropping and advanced persistent threats (APTs).
• Parental Controls: Parents can use firewalls to block their children from accessing explicit web content.
• Workplace Web Browsing Restrictions: Employers can restrict employees from using the company network to access certain services and websites, like social media.
• Nationally Controlled Intranet: Governments can block access to certain web content and services that conflict with national policies or values.

By allowing network owners to set specific rules, firewalls offer customizable protection for various scenarios, enhancing overall network security.

To maximize your firewall’s protection, enhance its security with these best practices:

Never turn off your firewall just to connect to a device or network. Instead, adjust your firewall rules and add trusted devices to your allow list.

Regularly update your firewall software or operating system to patch vulnerabilities and stop new security threats.

A VPN encrypts your internet traffic, adding another layer of protection alongside your firewall. Just be sure to adjust firewall rules if there’s a conflict.

If you get a suspicious access request, block it immediately. Investigate later before making any permanent changes.

Firewalls don’t block all threats especially malicious programs you install yourself. Use trusted antivirus or anti-malware software for full coverage.

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