What Is a Firewall? A Plain-English Guide for Small Business

The Security Guard for Your Network

If you've ever looked into cyber security for your business, you've almost certainly come across the word "firewall." It's one of those terms that gets thrown around a lot — but what does a firewall actually do, and do you already have one?

The simplest way to think about a firewall is as a security guard that sits between your business network and the internet, checking what's allowed in and what's allowed out. Just as a security guard at a building entrance checks credentials and turns away people who shouldn't be there, a firewall examines network traffic and blocks connections that look suspicious or unauthorised.

How a Firewall Works

The internet works by sending data in small packets between devices. Every time you load a webpage, send an email, or access a cloud application, data is travelling back and forth in these packets. A firewall inspects these packets and applies rules to decide which ones to allow through and which to block.

Basic firewalls work by checking things like:

• Where the data is coming from (the source IP address)
• Where it's going (the destination IP address)
• What type of traffic it is (the port and protocol)

More advanced firewalls — often called next-generation firewalls (NGFW) — go further. They can inspect the content of traffic, identify specific applications, detect and block malware, and make decisions based on the user's identity rather than just their device's address.

Types of Firewalls You Might Encounter

Hardware firewalls

A hardware firewall is a physical device that sits between your modem/router and your business network. Many business-grade routers include firewall functionality built in. Dedicated hardware firewalls from vendors like Cisco, Fortinet, or Sophos offer more advanced features. These are typically managed by an IT professional.

Software firewalls

A software firewall runs on individual computers and controls traffic to and from that specific device. Windows Firewall (now called Windows Defender Firewall) is built into every modern Windows PC and is turned on by default. macOS also has a built-in firewall, though it's not enabled by default — you'll need to turn it on in System Settings > Network > Firewall.

Cloud-based firewalls

As more businesses operate using cloud services and remote workforces, cloud-based firewalls (sometimes called Firewall-as-a-Service) have emerged. These inspect traffic going to and from cloud applications and can protect users regardless of where they're working. Vendors like Cloudflare and Zscaler offer these types of solutions.

Do You Already Have a Firewall?

Almost certainly — at least a basic one. If you're using a modern router (whether it's one provided by your ISP or one you've purchased), it almost certainly includes basic firewall functionality. Your Windows or Mac computers also have built-in software firewalls.

The more important question is whether these firewalls are properly configured. A firewall that's set to "allow everything" isn't providing much protection. Check that:

• Windows Defender Firewall is enabled on all Windows PCs (check in Windows Security settings)
• macOS Firewall is enabled on all Macs (check in System Settings > Network > Firewall)
• Your router's firewall functionality is enabled (check in your router admin interface)

What a Firewall Can and Can't Do

Firewalls are important, but they're not a complete solution on their own. Understanding their limitations helps you make better security decisions.

A firewall can:

• Block unauthorised incoming connections from the internet
• Prevent certain types of malware from communicating with command-and-control servers
• Control which services and ports are exposed to the internet
• Log network activity for monitoring and investigation

A firewall cannot:

• Stop malware that arrives via email attachments or malicious downloads
• Protect against phishing attacks (where a user is tricked into giving away credentials)
• Defend against attacks that use legitimate, allowed traffic as a cover
• Compensate for weak passwords or unpatched software

This is why cyber security professionals talk about "defence in depth" — using multiple layers of protection rather than relying on any single control. A firewall is an essential layer, but it works best alongside endpoint security software, MFA, regular patching, and staff awareness training.

When Should You Invest in a Better Firewall?

For a small business with a handful of computers and a basic internet connection, the built-in firewalls in your router and operating systems may be sufficient — provided they're properly configured and kept up to date. As your business grows, or if you handle particularly sensitive data (health information, financial records, payment card data), it's worth speaking with an IT professional about whether a more capable dedicated firewall solution is appropriate.

The Australian Signals Directorate's Essential Eight framework includes network segmentation and firewall controls as part of its recommended baseline for all Australian organisations. If your business is working towards Essential Eight compliance, a review of your firewall configuration is an important step.

Key Takeaways

• A firewall acts as a security guard between your network and the internet, filtering allowed and blocked traffic
• You likely already have basic firewalls — check that Windows Defender Firewall and macOS Firewall are both enabled
• Hardware, software, and cloud-based firewalls each have different roles and are often used together
• Firewalls are essential but not sufficient on their own — they work best as one layer of a broader security approach
• Ensure your router's built-in firewall is active and properly configured

Not sure whether your firewall setup is adequate for your business? The free assessment at flagged.com.au helps you identify gaps in your network security and prioritise what to address first.