In network security, what is the primary function of a firewall placed between two networks such as a LAN and the Internet?

Introduction / Context:
Firewalls are a core component of network security architectures. For Cisco and other networking certifications, you must understand what a firewall actually does, and also what it does not do by itself. This question asks for the primary function of a firewall deployed between two networks.

Given Data / Assumptions:

• A firewall is positioned between an internal trusted network and an external or less trusted network such as the Internet.
• The firewall is configured with security policies that define which traffic is allowed or denied.
• The question focuses on the main purpose, not on every advanced capability.

Concept / Approach:
At its core, a firewall inspects network traffic at one or more layers (for example, packet filtering, stateful inspection, or application layer inspection) and enforces security policies. These policies are expressed as rules that match on IP addresses, ports, protocols, and sometimes application content. The firewall decides whether each packet or flow should be permitted, denied, or subject to additional processing such as NAT or logging.

Step-by-Step Solution:
Step 1: Understand that traffic moving between two networks passes through the firewall as an intermediate device.Step 2: The firewall examines packet headers and, in some cases, packet payloads to determine which rule applies.Step 3: Based on the configured policy, the firewall permits, drops, or rejects the traffic. It may also perform state tracking to ensure that packets belong to valid sessions.Step 4: The firewall can log decisions, apply NAT, and integrate with intrusion prevention systems, but these are additional capabilities built on top of the basic permit or deny function.

Verification / Alternative check:
On real firewalls, commands such as show access-list, show security policy, or logs from blocked connections demonstrate that the firewall is continuously matching traffic against rules and deciding whether to forward or block packets. This behaviour directly supports the description in option A.

Why Other Options Are Wrong:
Option B describes simple address mapping or NAT functionality without security filtering, which is not the primary role of a firewall.Option C reduces the firewall to a pure antivirus scanner based on signatures, which is more typical of an antivirus or IDS or IPS device.Option D exaggerates firewall capabilities; no firewall can guarantee complete virus prevention without other layers such as endpoint protection and user training.

Common Pitfalls:
Many newcomers think that a firewall alone can solve all security problems, including malware and phishing. In reality, a firewall enforces network access control policies but must be combined with other tools. Recognizing that its fundamental job is to examine traffic and decide what to allow or deny keeps expectations realistic and aligns with exam objectives.

Final Answer:
The primary function of a firewall is that It examines packets and applies security rules to decide which traffic should be allowed or denied between networks.