GCSE · Computer Science · AQA · Spec 8525

Firewall

Data is arriving at a network's edge all the time, and leaving it too. Something has to decide what gets through. Right now, that's you.

Be the firewall

Allow or block?

You're the firewall: the network's checkpoint. Each piece of traffic below is either coming in or going out. Pick one, then put it in Allow or Block using only the rule book that's switched on. When all six are sorted, switch rule books and sort the same traffic again.

Rule book A. (1) Let out anything a computer inside sends. (2) Let in replies to something a computer inside asked for. (3) Block any other incoming traffic.

Still to sort

Allow (0)

A rule in the rule book that's switched on lets it through.

Block (0)

The rule book that's switched on says no.

Where the line is: How the traffic 'looks' doesn't decide this. The rules do. If they say block, it's blocked.

6 of 6 still to sort.

Sorted with both rule books? Count how many verdicts flipped, and check which direction each of those was travelling.

Why bother?

Reason it through

Why does a network need security at all?

Link 1 of 4

First link · your turn

Why do we join computers into a network in the first place?

Locked — reveal the link above first

What do you think now?

What is a firewall, really?

You've been the firewall. Which of these is closest to what you think a firewall is?

It checks traffic going in and coming out against its rules, and allows or blocks each piece.It's basically a virus scanner. It looks through files and removes viruses.It guards the way in. It stops bad stuff getting into the network.If a network has a firewall, it's completely secure.

How sure are you?

I'm sureI think soI'm guessing

It checks traffic going in and coming out against its rules, and allows or blocks each piece.

This matches the evidence

Yes. That's exactly what you were doing with the rule books: every piece of traffic, in either direction, checked against the rules, then allowed or blocked. The firewall doesn't decide what's 'good'. Whoever writes the rules decides that.

It's basically a virus scanner. It looks through files and removes viruses.

This is a common misconception

Finding and removing viruses in files on a computer is a different job, done by anti-malware software. What makes something a firewall is the traffic decision: it monitors traffic in and out and allows or blocks it using a defined set of security rules. The capabilities of firewalls have changed dramatically in recent years and will continue to do so, but that rule-based allow-or-block decision is the idea to hold on to.

It guards the way in. It stops bad stuff getting into the network.

This is a common misconception

Half right, which makes it easy to believe. A firewall checks outgoing traffic too. Remember rule book B: the file-transfer program sending files out was blocked on its way OUT. Checking outgoing traffic is how a network stops data leaving that shouldn't.

If a network has a firewall, it's completely secure.

This is a common misconception

A firewall's job is deciding what traffic is allowed in and out. Checking that a user is who they claim to be is authentication's job, and making intercepted data unreadable is encryption's. Using several methods together gives more security than any one of them alone.

A firewall checks traffic both ways against its rules. It's one important method, not the whole of network security.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

A firewall doesn't know good from bad. It follows the rule book someone gave it.

What you need to know

Networks need security: the links that let devices share data also let others try to reach them, risking unauthorised access, theft or damage to data.
A firewall is a network security device that monitors incoming and outgoing traffic.
It allows or blocks each piece of traffic based on a defined set of security rules.
The rules decide the verdict, so the same traffic can be allowed by one rule book and blocked by another.
A firewall is one of several methods, alongside authentication, encryption and MAC address filtering. Used together they give more security.

The big picture

A network needs security because the connections that let devices share data also let outsiders try to reach them. A firewall is one method of network security: it monitors traffic coming into and going out of a network and allows or blocks each piece using a defined set of security rules. It decides nothing by itself, so different rules give different verdicts for the same traffic. It works alongside authentication, encryption and MAC address filtering, each doing a different job, and together they give more security than any one alone.

Key points

1Direction matters: a firewall checks outgoing traffic as well as incoming, so it can stop data leaving the network.

2A reply to something a computer inside asked for can be let in, while a connection nobody asked for can be blocked. It depends on the rules.

3Authentication checks who a user is, encryption makes intercepted data unreadable, and MAC address filtering only lets approved devices join.

4No single method covers every risk, which is why networks combine them.

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Worked example

Problem

A family wants their home firewall to do three things: let everyone browse the web; stop any computer outside starting a connection to their devices; and stop the children's tablet connecting to online game servers. Write a rule book for the firewall, then use it to decide what happens when a web page the tablet asked for arrives back.

⚠ Watch out

Saying a firewall only stops things getting IN. It checks traffic going out of the network too, and blocks or allows it by the same kind of rules.

🧠

Memory hook

Both ways, by the book: in or out, the firewall checks the rule book, then allows or blocks.

✓

Check yourself

Cover the page. What does a firewall check, in which two directions, and what decides each verdict? Name one job it doesn't do, and the method that does.

Flashcards

(7)

What does a firewall do with each piece of network traffic?

Checks it against its defined set of security rules, then allows it or blocks it.

Which directions of traffic does a firewall monitor?

Both: incoming traffic and outgoing traffic.

Why might one firewall allow some traffic that another firewall blocks?

They have different rules. The verdict comes from the rules, not from the traffic itself.

Why does a network need security?

The connections that let devices share data also let others try to reach them, risking unauthorised access, theft or damage.

Which method checks that a user is who they claim to be?

Authentication, for example a username and password.

Encryption or MAC address filtering: which one makes intercepted data unreadable?

Encryption. MAC address filtering only controls which devices can join the network.

Why combine firewall, authentication, encryption and MAC address filtering?

Each does a different job, so together they cover more risks and give more security than any one alone.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Firewall

Allow or block?

What is a firewall, really?

Which method does the job?

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