What Does the New Firewall Allow?

About this interactive

Firewalls differ by brand, so the lesson uses OPNsense, popular and open source. Picture one with three interfaces: LAN 1, LAN 2 and the WAN. Each interface has an in direction, from its network into the firewall, and an out direction. OPNsense decides on the way in. It puts a restrictive access list in front of every in and leaves every out wide open. Out of the box, the WAN's guard stops anything that starts on the internet, so nobody outside can open a connection to anything inside. The first LAN gets a default rule that lets its devices go out to anywhere. Every interface added after it starts with no rules, so its devices can reach nothing until you write some. Because the firewall is stateful, it records each conversation LAN 1 starts, its addresses and ports, so the replies come back in without a rule of their own. Three reasons, then: LAN 1's default rule, the state table, or blocked until someone opens a hole.

What you'll learn

Predict how a newly installed stateful firewall treats traffic: the first LAN may go out by its default rule, replies come back through the state table, and anything arriving on the WAN or on a LAN with no rules is blocked
Explain why each interface's in direction is where the firewall decides, and why every LAN after the first needs rules added before it can reach anything

Aligned to

CompTIA Security+

4.5 Given a scenario, modify enterprise capabilities to enhance security.

3.2 Given a scenario, apply security principles to secure enterprise infrastructure.

Key terms

Stateful Firewall: A firewall that tracks the state of active network connections and makes filtering decisions based on context.
State Table: A data structure maintained by a stateful firewall that records active connection information, including connection status and expected next steps, to evaluate whether packets belong to a legitimate session.
Interface Direction: The orientation of traffic flow on a router interface, either inbound (into the interface) or outbound (out of the interface), which determines where an ACL is applied and evaluated.
Implicit Deny: A foundational access control principle in which any traffic or request not explicitly permitted by a rule is automatically blocked. It appears as a hidden deny-all rule at the end of every access control list.
Firewall: A network security device that monitors and controls incoming and outgoing traffic based on predefined security rules.

Topics

Stateful Firewall State Table Interface Direction Firewall Interactive Categorize

About TechKnowSurge

TechKnowSurge builds IT and cybersecurity professionals through hands-on, concept-first training built around real understanding — not memorization. Free interactive tools, structured programs, and 25+ years of real-world experience, all in one place.

Explore free tools and programs →