The Plain English Version

A firewall is a security barrier between your network and the internet. It monitors all incoming and outgoing traffic and blocks connections based on rules you set - like a security guard checking everyone who wants to enter or leave your building.

Without a firewall, anyone on the internet could potentially connect to your systems. With one, you control what gets in and what gets out.

Types of Firewalls

Network Firewall: Sits at the edge of your network, protecting everything behind it. Often a physical device or your router.

Host Firewall: Software running on individual devices (like Windows Firewall). Protects that specific device.

Next-Gen Firewall (NGFW): Advanced firewalls that inspect traffic content, not just addresses and ports.

What Firewalls Actually Do

  • Block unwanted connections - Stop attackers scanning for open services
  • Control outbound traffic - Prevent malware from calling home
  • Log network activity - Create records for investigation
  • Segment networks - Keep sensitive systems separated
  • Filter content - Block known malicious sites (NGFW)

Common Mistakes

Many businesses set up a firewall once and forget about it. Rules accumulate, nobody knows why certain ports are open, and the firewall becomes Swiss cheese. Regular review of firewall rules is essential.

Another mistake: assuming your router firewall is sufficient. Consumer routers provide basic protection but lack the logging, control, and inspection capabilities businesses need.

For SMEs

At minimum, ensure your router has its firewall enabled and default passwords changed. For better protection, consider a dedicated firewall appliance or unified threat management (UTM) device. Many MSPs can manage this for you.

Cyber Essentials requires a properly configured firewall - it is one of the five technical controls.