Understanding Firewalls

The Internet is a powerful and open system but that openness also creates risks. To keep networks secure, we rely on firewalls, the first line of defense against unwanted traffic, hackers, and cyberattacks.

What Is a Firewall?

A firewall is a security barrier that monitors and controls incoming and outgoing network traffic based on a set of rules. It acts as a filter between trusted and untrusted networks, such as between your computer and the Internet.

info

Think of a firewall as a security guard at the entrance of a building, checking IDs and only allowing authorized personnel to enter.

In simple terms, firewalls help ensure that only safe and approved data can pass through to your network.

How Firewalls Work

Firewalls inspect data packets as they travel across networks. Each packet is analyzed against security rules, such as:

Source and destination IP addresses
Port numbers
Protocols (HTTP, HTTPS, FTP, etc.)
Packet contents (in advanced firewalls)

Basic View
Technical View

A basic firewall might allow web traffic (port 80/443) but block suspicious connections or file transfers on other ports.

Example: Simple Firewall Simulation

Live Editor

function FirewallSimulator() {
  const handleRequest = (type) => {
    if (type === "http") alert("Allowed: Web traffic (Port 443)");
    else alert("Blocked: Unauthorized traffic (Port 23)");
  };
  return (
    <div style={{ textAlign: "center" }}>
      <h3>Firewall Traffic Filter</h3>
      <button onClick={() => handleRequest("http")}>Send HTTPS Request</button>
      <button onClick={() => handleRequest("telnet")}>Send Telnet Request</button>
    </div>
  );
}

Result

Types of Firewalls

Firewalls can operate at different layers of the network stack and offer varying levels of security:

Type	Layer	Description
Packet-Filtering Firewall	Network	Checks basic info like IPs and ports; fast but limited.
Stateful Inspection Firewall	Transport	Tracks active connections and allows related packets.
Proxy Firewall	Application	Intercepts and inspects data at the application layer (HTTP, FTP).
Next-Generation Firewall (NGFW)	Multiple	Includes intrusion detection, malware filtering, and deep inspection.
Cloud Firewall (FWaaS)	Cloud	Firewall-as-a-Service — protects cloud apps and virtual networks.

Firewall Rules Example

Rule	Action	Description
Allow TCP port 443	Allow	Enable secure web browsing (HTTPS).
Block TCP port 23	Block	Disable Telnet — an insecure protocol.
Allow ICMP from internal network	Allow	Permit internal ping requests.
Block all inbound traffic by default	Block	Enforce a default-deny security posture.

# Example Linux UFW firewall commands
sudo ufw default deny incoming
sudo ufw allow 443/tcp
sudo ufw deny 23/tcp
sudo ufw enable

Network Placement of Firewalls

Firewalls can exist in multiple forms hardware, software, or cloud-based — and are typically placed between the LAN and Internet. In a typical home or office setup:

Some organizations use multiple layers of firewalls perimeter firewalls at the network edge and internal firewalls between departments or services.

Stateful vs Stateless Firewalls

Feature	Stateless Firewall	Stateful Firewall
Tracks connections	No	Yes
Security level	Basic	High
Performance	Fast	Slightly slower
Use case	Simple traffic filtering	Complex enterprise networks

note

Stateful firewalls are generally preferred for modern networks due to their ability to monitor ongoing connections and provide enhanced security.

Firewall Limitations

While firewalls are powerful, they aren’t a complete solution on their own.

Cannot detect internal threats or phishing attacks.
May slow down traffic if poorly configured.
Need regular updates to remain effective.
Must be combined with antivirus, intrusion detection, and monitoring tools.

Real-World Examples of Firewalls

Vendor	Product	Highlights
Cisco ASA	Enterprise Firewall	Hardware-based security with advanced inspection.
Fortinet FortiGate	Unified Threat Management	Combines firewall, VPN, and intrusion prevention.
Palo Alto Networks NGFW	Next-Gen Firewall	Application-level inspection with ML-driven threat detection.
Cloudflare WAF	Cloud Firewall	Protects websites from online attacks at the edge.

Key Takeaways

A firewall is a traffic filter that protects your system from unauthorized access.
It uses predefined rules to allow or block network traffic.
Modern Next-Gen Firewalls combine inspection, intrusion prevention, and threat intelligence.
Firewalls are essential for network security, but should be part of a multi-layered defense strategy.

What Is a Firewall?​

How Firewalls Work​

Example: Simple Firewall Simulation​

Types of Firewalls​

Firewall Rules Example​

Network Placement of Firewalls​

Stateful vs Stateless Firewalls​

Firewall Limitations​

Real-World Examples of Firewalls​

Key Takeaways​