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What Is a Firewall? How It Works & What It Can't Do (2026)

A firewall monitors and filters network traffic by a set of rules, blocking unauthorized connections. What a firewall is, the main types, what it protects, and what it doesn't - it's not a VPN, encryption, or antivirus.

By Eric Gerard · Editor · AnonymFlow4 min readPhoto: Unsplash

You've seen "Windows Firewall" pop up, or toggled a firewall setting on your router, but what is it actually doing? A firewall is a gatekeeper for network traffic - it decides which connections are allowed and which are blocked. This guide explains what a firewall is, the main types, what it genuinely protects, and the common myths about what it can't do.

What a firewall is

A firewall is a security system - software, hardware, or both - that monitors network traffic and allows or blocks it based on rules. It sits between a trusted side (your device or home network) and an untrusted one (the internet), stopping unauthorized connections while letting legitimate traffic through.

You already use several: your OS has one, your router has one, and businesses run dedicated appliances. It's a foundational layer that controls which connections are permitted.

Server racks in a data center
Server racks in a data center

The main types

By technique:

  • Packet-filtering - checks each packet against rules (address, port, protocol).
  • Stateful - tracks connection state and allows return traffic for sessions you started.
  • Application-layer / next-gen (NGFW) - inspects content and can block specific apps or threats.

By location:

  • Host-based - runs on one device (Windows/macOS firewall).
  • Network - protects a whole network at the router or an appliance.

Most setups combine a network firewall with host firewalls on each device.

What it protects against

A firewall is a gatekeeper for connections:

  • Blocks unsolicited inbound attempts - port scans, attackers reaching for a service.
  • Can restrict which programs on your device reach the internet.
  • Enforces rules like blocking ports or destinations.

On a home router, it's why random machines on the internet can't directly connect to your devices. (For how connections actually find your devices, see what is DNS.)

The scale of what those rules cover is worth stating plainly. A port number is a 16-bit field, so there are 65,536 of them, numbered 0 to 65,535, and that whole range exists separately for TCP and for UDP. They fall into three bands: 0 to 1023 are the well-known ports, 1024 to 49151 the registered ones, and 49152 to 65535 the ephemeral range your own outgoing connections borrow from. A default-deny policy that allows only ports 80 and 443 is therefore shutting 65,534 of the 65,536 TCP doors, which is both why that setup is so common and why VPN traffic on port 443 slips through it.

A server room aisle lined with racks
A server room aisle lined with racks

What it can't do (the myths)

A firewall is widely overestimated. It doesn't:

  • Encrypt your traffic - anyone in between can still read unencrypted data. That's a VPN's job.
  • Hide your IP or location.
  • Stop malware you run yourself, or phishing that tricks you - that traffic looks legitimate.
  • Protect a connection it's set to allow.

A firewall governs what connects, not what's inside the traffic it permits.

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Firewall + VPN: complementary

They solve different problems, so use both. The firewall refuses unwanted connections (great against an attacker probing an open port). The VPN encrypts your traffic and hides your IP from the networks and sites you use (great on untrusted Wi-Fi - see public Wi-Fi risks). Neither replaces the other.

The bottom line

A firewall is a rule-based gatekeeper that allows or blocks network connections - built into your OS and router, essential for keeping unsolicited traffic out. But it's one layer: it doesn't encrypt, hide your IP, or stop malware and phishing you let through. Pair it with a VPN for encryption and IP privacy, plus anti-malware and good habits, for real defence in depth.

Editorial guide based on how firewalls work (packet-filtering, stateful, NGFW; host vs network) and their documented limits versus VPNs and anti-malware. Commercial links carry the rel="sponsored nofollow" attribute; an affiliate commission may apply at no extra cost to you.

Network security guides related to firewalls

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Everything you need to know.

Frequently asked questions

What is a firewall?

A firewall is a security system - software, hardware, or both - that monitors incoming and outgoing network traffic and decides what to allow or block based on a set of rules. It acts as a barrier between a trusted network (your device or home network) and untrusted ones (the internet), stopping unauthorized connections while letting legitimate traffic through. Every modern operating system includes one, your router has one, and businesses run dedicated firewall appliances. It's a foundational layer of network security, controlling which connections are permitted.

What are the main types of firewall?

By technique: packet-filtering firewalls check each packet against rules (source/destination, port, protocol); stateful firewalls track the state of connections and allow return traffic for sessions you started; and application-layer or next-generation firewalls (NGFW) inspect the actual content and can block specific apps or threats. By location: host-based firewalls run on a single device (like the one in Windows or macOS), while network firewalls protect a whole network at the router or a dedicated appliance. Most setups combine a network firewall with host firewalls on each device.

What does a firewall protect against?

A firewall mainly controls connections: it blocks unsolicited inbound attempts (port scans, attackers trying to reach a service), can restrict which programs on your device reach the internet, and can enforce rules like blocking certain ports or sites. On a home router, it's why random machines on the internet can't directly connect to your devices. It's a gatekeeper for network traffic - useful and important - but it governs what connects, not what's inside the traffic it allows.

What can't a firewall do?

Plenty, and this is widely misunderstood. A firewall doesn't encrypt your traffic - anyone between you and the destination can still read unencrypted data (that's a VPN's job). It doesn't hide your IP or location. It generally can't stop malware you invite in by downloading and running it, or phishing that tricks you into handing over credentials, since that traffic looks legitimate. And it can't protect a connection it's configured to allow. A firewall is one layer; it needs encryption, anti-malware and good habits alongside it.

Do I need a firewall if I use a VPN?

They do different jobs, so ideally yes to both. A firewall controls which connections are allowed in and out; a VPN encrypts your traffic and hides your IP from the sites and networks you use. A firewall won't encrypt anything, and a VPN won't block an attacker probing an open port on your device. On untrusted networks, the combination is strong: the firewall refuses unwanted connections, the VPN protects the content and origin of the traffic you do send.

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