What is a firewall?

What it is

An administrator writes the rules. A typical office set lets staff computers reach the internet, lets in only the few services the business publishes, and drops every other incoming connection.

Most firewalls today are stateful. That means the firewall remembers which connections your own computers started, lets the replies come back, and blocks traffic that nobody asked for.

A firewall can be a hardware appliance, or software running on standard hardware or in a virtual machine. An appliance is simple to buy and plug in. Software on standard hardware lets the business pick the machine and avoids being tied to one vendor. OPNsense and pfSense are examples of the software kind.

Why it matters to a business

• More than blocking: a business firewall usually adds VPN for remote staff and other offices, intrusion detection that flags known attack patterns, and content filtering that blocks categories of websites.
• Segmentation: VLANs split the network into zones, such as staff, guests, cameras and servers, and the firewall controls what each zone can reach.
• High availability: two firewalls can run as a pair, where the second takes over automatically if the first fails, so one dead box does not stop the office.
• Beyond a consumer router: a home router bundles a basic firewall with Wi-Fi and has little room for VLANs, detailed logs, many VPN users or failover.

How this shows up in our work

We build business networks on OPNsense firewalls in high-availability pairs, with VLAN segmentation, site-to-site VPN and Tailscale remote access. When a client already runs a firewall from another vendor, such as Fortinet, we support what is there before suggesting any change.

Our networking and firewalls page covers the build in more detail.

Learn more

• Firewall on WikipediaAn overview of firewall types and their history.
• Stateful firewall on WikipediaExplains how connection tracking works, for readers curious about what stateful inspection does.