How Computers Work · Your Computer on a Network · Lesson 4 of 5

The Firewall

What the firewall blocks, Private vs Public networks, and why it stays on.

What a firewall does

Your computer sends and receives data all day. Most of it is traffic you asked for, like a web page you opened. But other computers on a network can also try to reach yours without being asked.

A firewall is a guard that checks network traffic and blocks connections that aren't allowed. Think of it as a doorman. People you invited get in. Strangers who just knock are turned away.

The key idea is direction:

• Outgoing traffic starts on your computer, like your browser asking for a page. The replies to these requests are let back in.
• Incoming traffic starts somewhere else and tries to reach your computer. The firewall blocks unexpected incoming connections unless a rule allows them.

Windows 11 has a firewall built in, called Microsoft Defender Firewall. It's on by default. To check it:

1. Open the Windows Security app from Start.
2. Choose Firewall & network protection.
3. You'll see three network types, Domain, Private and Public, each saying whether the firewall is on.

Domain is for work computers run by a company. At home you'll mostly see Private and Public. The one marked (active) is the type you're using now.

A firewall is not a cure for everything. It doesn't stop a scam email or a bad download you open yourself. It guards the door, not what you carry in.

On a Mac, the firewall is in System Settings > Network > Firewall. It may be off at first, and you can turn it on there.

On the practice computer, check that the firewall is on for Private and Public networks. When an app asks "Allow on public networks?", choose Private only, then watch the firewall block an unexpected incoming connection.

Private, Public and the app prompt

Windows treats networks differently depending on how much you trust them. Each Wi-Fi or Ethernet network gets a network profile, a label that sets how open your computer is on it.

• Public network: your computer stays hidden from other devices on the network. Use this for cafes, airports, hotels and libraries. Microsoft recommends Public for most networks, even at home.
• Private network: other devices can find your computer, so you can share files or a printer. Only use it on a network where you trust every device.

New networks start as Public. To change one, open Settings > Network & internet > Wi-Fi, choose your network, then pick under Network profile type.

Sometimes an app, like a game or video chat program, wants other devices to reach it. Windows Security then shows a prompt. It asks whether to let the app communicate on Private networks, Public networks, or both. A good habit:

• If you trust the app and it needs the network, allow Private only.
• Leave Public unchecked. Public networks are full of strangers.
• If you don't recognize the app, choose Cancel. You can usually allow it later if something stops working.

Now the most important rule: leave the firewall on. Some websites or helpers say "turn off your firewall" to fix a problem. That opens every door at once. If an app truly needs access, allow that one app instead.

Watch out for anyone who calls you and asks you to turn it off. That's a common trick in tech support scams.