What Is a Firewall? And Do You Already Have One?

Something has told you that you need one. A security suite offering a trial, an advert, or a warning that your PC is unprotected. Before you pay for anything, there is a fact worth knowing.

You already have a firewall. It came with Windows, it is switched on right now, and Microsoft says so in writing.

So this page is about what it actually does, what it does not do, and whether you need anything else.

You already have one, and it is already on

Documented by Microsoft in its Windows Firewall overview, in one sentence: "Windows Firewall is a host-based firewall that is included with the operating system and enabled by default on all Windows editions."

All editions. Home included. And enabled without you doing anything.

Microsoft makes the commercial point too: it is "included with the operating system, so no additional hardware or software is required".

Ours, and marked as ours: so a warning telling you that your PC has no firewall is either wrong or is talking about its own product rather than the one you have.

What it actually blocks, and this surprises people

Now the part almost nobody knows, and it changes what you should expect from it.

Documented by Microsoft, the default behaviour of Windows Firewall is to "block all incoming traffic, unless solicited or matching a rule" and to "allow all outgoing traffic, unless matching a rule".

⭐ Read the second half. Everything coming IN is blocked unless you asked for it. Everything going OUT is allowed.

So a firewall stops something on the internet reaching into your machine uninvited. It does not stop a program on your machine reaching out.

Ours, and marked as ours: that is why a firewall is not protection against something already installed. If malware is on your PC and phoning home, the firewall lets it, because it is outgoing. People assume the opposite and it matters.

What it is genuinely for

Blocking the uninvited.

Microsoft describes the mechanism: it filters traffic "based on several criteria, including source and destination IP address, IP protocol, or source and destination port number".

In plain terms: something on the internet knocks on a door on your machine, and unless a program on your side asked for that connection, the firewall does not answer.

⭐ That is a real and constant job. Machines on the open internet are scanned continuously by automated tools looking for open doors. The firewall is why none of them find one.

The three network profiles, and the one that matters

Documented by Microsoft: Windows Firewall has three profiles, "domain, private and public", and rules can differ between them.

Public is "designed with higher security in mind for public networks, like Wi-Fi hotspots, coffee shops, airports, hotels". Crucially, Microsoft notes it is "the default profile for unidentified networks".

Private is "designed for private networks such as a home network" and is set by hand.

Domain applies automatically on a work network with a domain controller, and cannot be set manually.

⭐ This is why file sharing works at home and not in a café, and why answering "yes, this is a private network" on a hotel Wi-Fi prompt is worth not doing. That prompt is choosing your firewall profile.

Do you need to buy another one?

Microsoft's own answer, in its own words: it recommends that you do not disable Windows Firewall, and it notes that Windows Firewall "is also designed to complement existing non-Microsoft network security solutions through a documented API".

Ours, and marked as ours: for a home PC, the built-in firewall does the job it exists to do. A paid suite adds other things, some useful, but a second firewall is not one of the useful ones.

If you buy a suite, that is a decision about its other features rather than about firewalls.

The warning about turning it off

⛔ This is the most practically useful thing on the page and hardly anybody knows it.

There is a wrong way to disable Windows Firewall, and it breaks things that look completely unrelated.

Documented by Microsoft: do not disable it "by stopping the Windows Firewall service". Stopping that service "isn't supported by Microsoft and can cause problems, including:"

• "Start menu can stop working"
• "Modern applications can fail to install or update"
• "Activation of Windows via phone fails"
• "Application or OS incompatibilities that depend on Windows Firewall"

⭐ A broken Start menu, caused by stopping a firewall service. If you have ever seen advice to disable that service for performance, that is what it costs.

Microsoft's own line on the right way, if it must be done: "The proper method to disable the Windows Firewall is to disable the Windows Firewall Profiles and leave the service running."

When something you need is blocked

The instinct is to turn the firewall off. Microsoft's instruction is the opposite.

Documented by Microsoft: "If there's an app you need to use that's being blocked, you can allow it through the firewall, instead of turning the firewall off."

In the Windows Security app, under Firewall & network protection, there is an option to allow an app through. Add the one app rather than opening everything.

If the app is failing rather than being blocked, is the service down or is it you has three tests that settle it in two minutes.

What a firewall is not

It is not antivirus. It controls connections. It does not inspect files or detect malware. Those are separate jobs and Windows does both, in different places.

It is not a VPN. A firewall decides what may connect. A VPN changes the route your traffic takes. What a VPN actually hides covers that, including a regulator's own warning about what it does not do.

It is not a spam filter, a tracker blocker or a parental control. Those all get sold in the same box and none of them is a firewall.

Status: written 22 August 2026. Microsoft documents Windows Firewall as enabled by default on all editions, and warns that stopping its service can break the Start menu.

Do I need a firewall if I have Windows?

You already have one. Microsoft states that Windows Firewall is "included with the operating system and enabled by default on all Windows editions". It also says no additional hardware or software is required, and recommends against disabling it.

Does a firewall stop viruses?

No. A firewall controls which connections are allowed. It does not inspect files or detect malware. It also allows all outgoing traffic by default, so it does not stop something already on your PC from sending data out.

Should I set my network to public or private?

Public on anything you do not control: hotels, cafés, airports. It is Microsoft's default for unidentified networks and it applies tighter rules. Private is for your own home network, where you want file sharing and device discovery to work.

Is it safe to turn off Windows Firewall?

Microsoft recommends against it, and warns specifically against stopping the Windows Firewall service. Doing that can stop the Start menu working, break app installs and updates, and cause activation to fail. Allowing one app through is the supported approach.

The Short Version

• Windows Firewall is on by default on every edition, including Home. Microsoft says so.
• It blocks everything coming in and allows everything going out. That is the default.
• So it does not stop something already on your PC from sending data out.
• Public profile is the default for unknown networks and applies tighter rules.
• Stopping the firewall service can break your Start menu. Microsoft documents that.
• If an app is blocked, allow that one app instead of turning the firewall off.

Where to Next

Open Windows Security and look at Firewall and network protection before you buy anything. You will find three profiles, all switched on, doing the job somebody is trying to sell you.

If you were also looking at a VPN, what a VPN actually hides quotes a regulator on what it does and does not do for you.

And if something on your machine is genuinely being blocked, is the service down or is it you has three tests that settle it in two minutes.

If a security suite told you that you had no firewall, I would like to know which one in the comments. That claim is worth collecting.