Firewall
Security system that monitors and filters network traffic by rules. Firewalls protect internal networks from unauthorized access and attacks.
The firewall is the first line of defence for every corporate network. It decides which traffic is allowed and which is blocked – like a gate that only lets authorized visitors in. From simple packet filters to AI-powered next-generation firewalls, the technology has evolved to keep up with increasingly sophisticated threats.
What is Firewall?
A firewall is a network security system that monitors and filters incoming and outgoing traffic according to defined rules. Firewalls sit between trusted internal networks and untrusted external networks (the internet). Types include: packet-filter firewalls (Layer 3/4: IP addresses and ports), stateful firewalls (track connection state), application-layer firewalls / WAF (Layer 7: HTTP etc.), and next-generation firewalls (NGFW: combine the above with IPS, deep packet inspection and AI). Firewalls can be hardware appliances, software or cloud services.
How does Firewall work?
A firewall inspects every packet crossing the network boundary: source IP, destination IP, port, protocol and with NGFW also payload. Rules decide: allow, block or log. Stateful inspection remembers established connections and allows related return traffic. Deep packet inspection (DPI) looks for malware, exploits and policy violations. Web application firewalls (WAF) sit in front of web apps and protect against OWASP Top 10 (SQL injection, XSS, CSRF). Cloud firewalls (e.g. Cloudflare, AWS Security Groups) protect cloud infrastructure.
Practical Examples
Enterprise NGFW: A Fortinet FortiGate filters all corporate internet traffic, detects malware via DPI and blocks known threats in real time.
Cloud WAF: Cloudflare WAF protects a web app from SQL injection, XSS and DDoS – no own hardware, global edge network.
AWS Security Groups: Each EC2 instance has a firewall allowing only explicitly permitted traffic on defined ports (deny by default).
Micro-segmentation: Host firewalls protect each system inside the network – limits lateral movement by attackers.
Typical Use Cases
Perimeter security: Separating internal network from the internet with controlled access
Web application protection: WAF in front of sites and APIs against web attacks
Network segmentation: Separating dev, production and guest Wi-Fi
Cloud security: Security groups and NACLs for cloud resources
Zero Trust: Micro-segmentation so every system has its own firewall
Advantages and Disadvantages
Advantages
- Core protection against unauthorized access and known threats
- Visibility: Logging and monitoring show who accessed what and when
- Compliance: Firewalls are required by many standards (ISO 27001, PCI-DSS)
- Flexible rules: Fine-grained control over allowed and blocked traffic
- Cloud options: Managed firewall services remove hardware and maintenance
Disadvantages
- Not a silver bullet: Firewalls do not protect against social engineering, insiders or zero-days
- Misconfiguration: Too permissive rules create gaps; too strict blocks legitimate traffic
- Performance: DPI and SSL inspection can reduce throughput
- Complexity: Enterprise firewalls with thousands of rules need management tooling
- Encrypted traffic: End-to-end encrypted traffic cannot be inspected without SSL decryption
Frequently Asked Questions about Firewall
Do I need a hardware firewall or is software enough?
What is the difference between a firewall and a WAF?
How do I configure a firewall correctly?
Related Terms
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