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Screened Subnet Firewall

Definition of Screened Subnet Firewall

A screened subnet firewall, also known as a dual-homed gateway or a screened host firewall, is a network security architecture that employs multiple layers of protection to safeguard an organization's internal network from external threats.

Origin of Screened Subnet Firewall

The concept of a screened subnet firewall emerged in the early days of networking as a response to the increasing need for robust security measures. It was pioneered by Marcus Ranum in the early 1990s as a solution to mitigate the vulnerabilities associated with single-homed gateway firewalls. Ranum's work laid the foundation for modern network security practices, emphasizing the importance of segregating internal networks from untrusted external networks.

Practical Application of Screened Subnet Firewall

A practical application of a screened subnet firewall is in the design of a demilitarized zone (DMZ), a network segment that acts as a buffer zone between the internet and an organization's internal network. In this setup, the screened subnet firewall sits between the internet-facing router and the servers hosting public-facing services such as web servers, email servers, and DNS servers. By filtering and inspecting incoming and outgoing traffic at the firewall boundary, organizations can enforce security policies and protect sensitive internal resources from unauthorized access.

Benefits of Screened Subnet Firewall

1. Enhanced Security: By implementing a screened subnet firewall, organizations can establish a secure perimeter that controls the flow of traffic between internal and external networks, reducing the attack surface and minimizing the risk of unauthorized access and data breaches.

2. Granular Control: Screened subnet firewalls allow administrators to define and enforce access policies based on factors such as IP addresses, port numbers, and application protocols, enabling granular control over network traffic and ensuring that only legitimate connections are permitted.

3. Scalability: The modular design of screened subnet firewalls facilitates scalability, allowing organizations to adapt their security infrastructure to evolving threats and network requirements by adding or reconfiguring firewall components as needed.

4. High Availability: By deploying redundant firewall components in a screened subnet configuration, organizations can achieve high availability and fault tolerance, ensuring continuous protection against network disruptions and minimizing downtime.


Unlike single-homed gateway firewalls, which have only one network interface connected to both internal and external networks, a screened subnet firewall features separate interfaces for internal and external connections, enhancing security by isolating internal resources from external threats.

A screened subnet firewall inspects incoming and outgoing traffic using access control lists (ACLs), stateful packet inspection, and other security mechanisms to enforce security policies and filter out malicious or unauthorized traffic before it reaches the internal network.

Screened subnet firewalls can be adapted to suit the needs of organizations of all sizes. While larger enterprises may require more complex configurations to accommodate their network infrastructure, small businesses can benefit from the enhanced security and control provided by a basic screened subnet firewall setup.


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