Firewall Rule Logic Simulator

Stateless firewalls evaluate each packet independently without knowledge of connection state. Stateful firewalls track active connections and automatically allow return traffic for established sessions. Stateful inspection is more secure because it prevents attackers from sending unsolicited packets disguised as return traffic, and it simplifies rule management since you only need rules for initiating traffic.

A shadowed rule is one that can never be matched because a broader rule above it already catches all the same traffic. For example, if rule #1 allows all TCP traffic from any source, then rule #5 that denies TCP port 22 is shadowed and will never trigger. Shadowed rules indicate a misconfiguration and should be reordered or removed.

The implicit deny all (also called "default deny" or "cleanup rule") is the last rule in a firewall ruleset that denies any traffic not explicitly permitted by previous rules. This is a security best practice that ensures only explicitly authorized traffic is allowed through the firewall, following the principle of least privilege.

A DMZ (demilitarized zone) typically allows inbound HTTP/HTTPS from external networks, permits specific connections from the DMZ to internal resources (like database servers), and blocks all cross-zone traffic by default. Rules should be organized with the most specific allows first, followed by inter-zone deny rules, with the implicit deny catching everything else.

CIDR (Classless Inter-Domain Routing) notation like 10.0.0.0/24 specifies a network range. The /24 means the first 24 bits are the network portion, allowing 256 addresses (10.0.0.0-10.0.0.255). Firewall rules use CIDR to match ranges of IP addresses instead of listing individual IPs, making rules more manageable and efficient.

Egress filtering controls outbound traffic from your network. Without it, compromised hosts can freely communicate with command-and-control servers, exfiltrate data, or participate in DDoS attacks. Best practices include allowing only necessary outbound ports (80, 443, 53), blocking direct outbound SMTP to prevent spam, and logging denied egress traffic for threat detection.

Firewall concepts are primarily covered in CISSP Domain 4: Communication and Network Security. This domain includes network architecture, secure network components, network access control, and secure communication channels. Understanding firewall rule logic, stateful inspection, and DMZ architecture are all key topics for the CISSP exam.