Network segmentation involves dividing a network into smaller, isolated segments to control and limit the flow of traffic between devices, systems, and applications. By segmenting networks, organizations can reduce the attack surface, restrict lateral movement by adversaries, and protect critical assets from compromise. Effective network segmentation leverages a combination of physical boundaries, logical separation through VLANs, and access control policies enforced by network appliances like firewalls, routers, and cloud-based configurations.

• Identify and group systems based on their function, sensitivity, and risk. Examples include payment systems, HR databases, production systems, and internet-facing servers.
• Use VLANs, firewalls, or routers to enforce logical separation.

• Host web servers, DNS servers, and email servers in a DMZ to limit their access to internal systems.
• Apply strict firewall rules to filter traffic between the DMZ and internal networks.

• In cloud environments, use VPCs, subnets, and security groups to isolate applications and enforce traffic rules.
• Apply AWS Transit Gateway or Azure VNet peering for controlled connectivity between cloud segments.

• Use software-defined networking (SDN) tools to implement workload-level segmentation and prevent lateral movement.

• Apply Access Control Lists (ACLs) to network devices to enforce "deny by default" policies.
• Use firewalls to restrict both north-south (external-internal) and east-west (internal-internal) traffic.

• Regularly review firewall rules, ACLs, and segmentation policies.
• Monitor network flows for anomalies to ensure segmentation is effective.

• Perform periodic penetration tests to verify that unauthorized access is blocked between network segments.

Use intrusion detection signatures to block traffic at network boundaries.

Employ network appliances and endpoint software to filter ingress, egress, and lateral network traffic. This includes protocol-based filtering, enforcing firewall rules, and blocking or restricting traffic based on predefined conditions to limit adversary movement and data exfiltration.

• Use Case: Configure network firewalls to allow traffic only from authorized IP addresses to public-facing servers.
• Implementation: Limit SSH (port 22) and RDP (port 3389) traffic to specific IP ranges.

• Use Case: Use firewalls or endpoint security software to block unauthorized outbound traffic to prevent data exfiltration and command-and-control (C2) communications.
• Implementation: Block outbound traffic to known malicious IPs or regions where communication is unexpected.

• Use Case: Restrict the use of specific protocols that are commonly abused by adversaries, such as SMB, RPC, or Telnet, based on business needs.
• Implementation: Disable SMBv1 on endpoints to prevent exploits like EternalBlue.

• Use Case: Create network segments for critical systems and restrict communication between segments unless explicitly authorized.
• Implementation: Implement VLANs to isolate IoT devices or guest networks from core business systems.

• Use Case: Use proxy servers or Web Application Firewalls (WAFs) to inspect and block malicious HTTP/S traffic.
• Implementation: Configure a WAF to block SQL injection attempts or other web application exploitation techniques.