← ACARA Digital Technologies Years 9–10

Firewall rule: allow or deny by port

A firewall rule with an action (allow), a direction (inbound) and a port (TCP 443). It lets incoming web traffic through. A deny rule on a port, such as 23, blocks it.

Part of: ACARA Digital Technologies Years 9–10 — Networked Systems and Access Control — Module 124, WA Computer Science ATAR Year 11 — Network Communications — Module 114, WA Computer Science ATAR Year 11 — Cyber Security — Module 119

Understanding Firewall rule: allow or deny by port in ACARA Digital Technologies Years 9–10

What it does: Outbound means traffic leaving the device. Allowing TCP 443 in that direction lets the student's browser reach secure websites. If a service is not needed, a deny inbound rule on its port closes that way in and gives attackers fewer openings. Open ports that nothing uses give attackers extra ways in. Closing unused ports shrinks the attack surface. A port rule only affects the port it names. This rule blocks port 23 but does not block other ports such as 443. A complete rule needs an action, a direction and a port. Without the direction, the firewall cannot tell which traffic the rule applies to.

Common mistakes: A web server must accept HTTPS traffic. Port 443 should be allowed, so the line should be allow inbound TCP 443. Denying it would block every secure visitor. The comment says Telnet (port 23) must be blocked. The action should be deny, so the line should be deny inbound TCP 23. Allowing it leaves an insecure port open.

Class: _____________________  ·  Name: _____________________  ·  Date: _________

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This module's key terms are also in a printable crossword (Modules 116–154) and word search (Modules 116–225).

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Learning
Duo
Which firewall rule lets secure web traffic (HTTPS) come into a school web server?
deny inbound TCP 443allow inbound TCP 443
Matches
allow TCP 443
↓
The rule has no direction. A firewall rule must say inbound or outbound so it is clear which way the traffic travels. It should be allow inbound TCP 443.
Applying
Meanings
Term
allow inbound TCP 443
Meaning
A firewall rule with an action (allow), a direction (inbound) and a port (TCP 443). It lets incoming web traffic through. A deny rule on a port, such as 23, blocks it.
Practice
allow inbound TCP 443
Testing
Gaps
inbound TCP 443
Check Answer
Spots
2allow inbound TCP 80
3deny inbound TCP 443

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Quick Questions

1. Which firewall rule lets secure web traffic (HTTPS) come into a school web server?
A. allow inbound TCP 443 ✓
B. deny inbound TCP 443
HTTPS uses port 443. An allow rule lets that traffic in. A deny rule would block students from reaching the secure site.
2. A firewall rule has three parts. Which list names them correctly?
A. Action, direction, port ✓
B. Username, password, port
A firewall rule states an action (allow or deny), a direction (inbound or outbound) and a port number, such as allow inbound TCP 443.
3. Which rule blocks Telnet (port 23) traffic coming in from the internet?
A. allow inbound TCP 23
B. deny inbound TCP 23 ✓
Telnet is old and sends data unencrypted. Denying inbound TCP 23 closes that port to outside connections.
4. What does 'inbound' mean in a firewall rule?
A. Traffic coming into the network from outside ✓
B. Traffic leaving the network to the internet
Inbound means incoming traffic. Outbound means traffic leaving your network. The direction tells the firewall which way the rule applies.
5. Which rule lets students on the school network visit websites over HTTPS?
A. deny outbound TCP 443
B. allow outbound TCP 443 ✓
Browsing starts from inside the network, so the traffic goes outbound to port 443. Denying it would stop students loading secure sites.

Match the Pairs

allow inbound TCP 443 → Let incoming HTTPS web traffic through the firewall on port 443
deny inbound TCP 23 → Block incoming Telnet connections on port 23
A rule that lets your computer send web requests out to the internet over HTTPS → allow outbound TCP 443
Closing an unused port by blocking incoming traffic to it, such as port 21 for FTP that nobody uses → deny inbound TCP 21
allow TCP 443 → The rule has no direction. A firewall rule must say inbound or outbound so it is clear which way the traffic travels. It should be allow inbound TCP 443.
deny inbound TCP → The rule has no port number, so it does not say which port to close. It should be something like deny inbound TCP 23.

Key Term

allow inbound TCP 443
A firewall rule with an action (allow), a direction (inbound) and a port (TCP 443). It lets incoming web traffic through. A deny rule on a port, such as 23, blocks it.

Fill in the Blanks

1. allow inbound TCP 443
Hint: Type the action word that lets matching traffic through the firewall.
2. allow inbound TCP 443
Hint: Type the direction word for traffic coming in from outside the network.
3. deny inbound TCP 23
Hint: Type the action word that blocks matching traffic.
4. allow outbound TCP 443
Hint: Type the port number used for secure web (HTTPS) traffic.
5. deny inbound TCP 23
Hint: Type the port number of the unused Telnet service that this rule closes.

Spot the Error

Question 1
A # Web server firewall rules
B allow inbound TCP 80
C deny inbound TCP 443
D deny inbound TCP 23
Line C — A web server must accept HTTPS traffic. Port 443 should be allowed, so the line should be allow inbound TCP 443. Denying it would block every secure visitor.
Question 2
A # Block Telnet from outside
B allow inbound TCP 23
C allow inbound TCP 443
Line B — The comment says Telnet (port 23) must be blocked. The action should be deny, so the line should be deny inbound TCP 23. Allowing it leaves an insecure port open.
Question 3
A default allow inbound
B allow inbound TCP 443
C allow inbound TCP 80
Line A — A default of allow would leave every unused port open. The first line should be default deny inbound, so only the ports with allow rules can be used.
Question 4
A # Staff PC: browse the web
B allow outbound TCP 443
C allow outbound TCP 80
D deny outbound 23 TCP
Line D — A rule is written in the order action, direction, protocol, then port number. The line should be deny outbound TCP 23, with the protocol before the port.
Question 5
A web: allow inbound TCP 23
B telnet: deny inbound TCP 23
C ftp: deny inbound TCP 21
Line A — Web traffic over HTTPS uses port 443, not 23 (which is Telnet). The line should be web: allow inbound TCP 443.

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