What Is A Firewall?
Article (PSA‑0006)
A firewall is a security barrier that separates a **trusted** network (your home or business LAN) from an **untrusted** network (the Internet). The term comes from fire‑prevention: just as a special type of wall can slow/stop a fire from spreading, a firewall can stop unwanted traffic from reaching your local devices.
Types of firewalls
- Software firewall – Runs on an operating system (Windows Defender Firewall, macOS Application Firewall, Linux iptables/nftables, third‑party tools).
- Hardware firewall – A dedicated appliance or a router with built‑in firewall capabilities.
- Cloud‑based firewall (FWaaS) – Managed firewalls delivered as a service (e.g., Azure Firewall, AWS Network Firewall, Cloudflare Zero Trust Gateway).
- Next‑Generation Firewall (NGFW) – Combines traditional packet filtering with deep‑packet inspection, intrusion‑prevention, application control, and often integrated sandboxing.
How Does a Firewall Work?
Firewalls enforce a set of security rules that decide whether to allow or block traffic. They inspect traffic at different layers of the OSI model, using one or more of the following methods.
Core filtering methods
- Packet‑filtering (Layer 3/4) – Examines source/destination IP addresses, ports, and protocol type. Simple, fast, but limited to “allow/deny” rules.
- Stateful inspection (Layer 4) – Tracks each connection’s state (SYN, SYN‑ACK, ESTABLISHED, FIN). Only packets that belong to a legitimate, established session are permitted, which reduces spoofing.
- Proxy / Application‑level gateway (Layer 7) – Acts as an intermediary for specific protocols (HTTP, SMTP, FTP). It can inspect the actual payload, enforce URL filtering, and hide internal IP addresses.
- Deep‑packet inspection (DPI) & sandboxing (NGFW) – Analyzes the full content of packets, detects malware, blocks suspicious scripts, and enforces application‑specific policies (e.g., “block Facebook app traffic”).
Do You Need a Firewall?
Absolutely. If a device can reach the Internet, it should be protected by an up‑to‑date firewall.
- Windows 10/11, macOS, and most Linux distributions include a built‑in host‑based firewall that is enabled by default.
- Home routers / ISP‑provided gateways usually have a basic hardware firewall (NAT + simple packet filter). You can tighten those rules via the router’s web UI.
- Small‑business networks benefit from a dedicated firewall appliance or a cloud‑based firewall that offers:
- Intrusion Prevention System (IPS)
- Application‑aware filtering
- VPN termination (site‑to‑site and client‑to‑site)
- Centralized logging & reporting
Quick self‑check
- Is the host‑based firewall turned on? (Windows → “Windows Defender Firewall”; macOS → “Firewall” in Security & Privacy.)
- Does your router have the default “block inbound unsolicited traffic” rule enabled?
- Are any ports deliberately opened to the Internet (e.g., remote‑desktop, web server)? If so, confirm they are necessary and secured (use VPN, strong passwords, MFA).
- Do you have a separate hardware or cloud firewall for your business network? If not, consider adding one as you scale.
Best‑Practice Checklist for Firewall Management (2025)
- Keep firmware and definitions up to date. Automatic updates are essential to protect against newly discovered exploits.
- Apply the principle of least privilege. Only allow traffic that is explicitly required; block everything else by default.
- Use separate zones. Separate Wi‑Fi guests, IoT devices, and corporate workstations into different VLANs/sub‑nets with inter‑zone firewall rules.
- Enable logging and alerts. Forward logs to a SIEM or a cloud log service to spot abnormal patterns early.
- Deploy a VPN or Zero‑Trust Network Access (ZTNA) solution. Remote users should never connect directly to the LAN; tunnel through a secure gateway.
- Regularly review rules. Conduct a quarterly rule‑audit; remove stale or overly permissive entries.
- Test your configuration. Use tools such as Nmap or external port‑scan services to verify that only intended ports are open.
- Consider a Next‑Generation Firewall. For businesses, NGFWs provide integrated IPS, URL filtering, and application control, reducing the need for multiple separate security products.
Emerging Trends (2025 Addendum)
- AI‑assisted threat detection – Modern NGFWs use machine learning to spot anomalous traffic patterns and automatically quarantine suspicious sessions.
- Zero‑Trust Network Access (ZTNA) – Replaces traditional perimeter firewalls with identity‑centric policies; each request is verified before access is granted.
- Secure Access Service Edge (SASE) – Converges firewall, secure web gateway, and cloud‑based security into a single, globally distributed service.
- Immutable, cloud‑native firewalls – Provide auto‑scaling, instant provisioning for hybrid‑cloud environments (Azure Firewall Premium, Google Cloud Armor).
Need a Firewall Review or Upgrade?
If you’re unsure whether your current firewall is configured correctly, or simply need a quick health‑check, give PSA Computer Services a call at (707) 506‑6802. We’ll assess your environment, and ensure you have a modern, reliable protective barrier.
