What is Default Deny Networking?

Default deny networking is a security posture where all network traffic is blocked unless explicitly permitted by a security policy. This is the opposite of "default allow," where traffic flows freely unless specifically blocked. Levels of default deny implementation: 1. **Firewall default deny**: All inbound traffic blocked; rules permit specific ports/IPs. Infrastructure is still visible to scanners — they receive RST or ICMP unreachable responses. 2. **ZTNA default deny**: Applications hidden behind a broker; access granted per-session after identity verification. Infrastructure may still be discoverable at the network layer. 3. **Network-layer default deny**: Infrastructure has zero network presence. No ports respond. No ICMP responses. No DNS records. Unauthorized traffic receives no response of any kind — the host appears non-existent. True default deny at the network layer is the strongest form because it eliminates the information that attackers need to plan and execute attacks. When a host appears non-existent, there is nothing to target. NIST SP 800-207 (Zero Trust Architecture) recommends default deny as a foundational principle, but most implementations only achieve Level 1 or 2.