Intermediate level · Module 1: Network Security Fundamentals · Lesson 1 of 3
Firewalls, Ports and Services
Understand how firewalls decide which traffic to allow.
In this lesson you will
- Explain ports, services and firewall rules
- Apply default-deny thinking
Key concepts
portsservicesinbound and outbound rulesdefault deny
The lesson
Network services listen on ports — web on 443, email submission on 587, remote desktop on 3389. Every open, reachable service is something an attacker can try.
A firewall filters traffic by rules. Good practice is default deny for inbound traffic: block everything, then allow only what is needed, from where it is needed.
Use both the router firewall and the host firewall on each computer (such as Windows Defender Firewall). Never expose remote desktop directly to the internet.
Step by step
- List services a small office needs.
- Write inbound rules with default deny.
- Check the host firewall is on.
- Document each rule's purpose.
Practice activity
Write and document a firewall rule set for a small office.
Expected result: A minimal, documented rule set with a reason for every allowed service.
Check your understanding
-
What does default deny mean?
Show answer
Block all traffic except what is explicitly allowed.
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Why should remote desktop not be exposed directly to the internet?
Show answer
It is a common target for password guessing and exploits.
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