Access Control List (ACL)

Definition

An Access Control List (ACL) is a list of rules that define which traffic is allowed or denied access to a network resource. ACLs are implemented on networking devices like routers, firewalls, and switches to control data flow and enforce security policies.

How ACLs Work

ACLs contain a set of rules, each specifying criteria for matching network packets. These criteria can include:

• Source IP Address: The IP address where the traffic originates.
• Destination IP Address: The IP address where the traffic is headed.
• Protocol: The type of network protocol (e.g., TCP, UDP, ICMP).
• Port Numbers: Specific ports used for services (e.g., port 80 for HTTP web traffic).

When a packet arrives, the networking device compares it against the ACL rules in sequential order. If a match is found, the corresponding action (permit or deny) is taken. If no match is found, a default action (usually to deny) is applied.

Benefits of ACLs

• Security: ACLs act as a first line of defense, preventing unauthorized access to sensitive systems or network segments.
• Traffic Management: Control the flow of specific types of traffic, prioritizing critical applications or limiting bandwidth usage.
• Resource Protection: Protect servers and devices from unnecessary traffic or potential attacks.

Common Use Cases

• Firewall Rules: ACLs form the basis of firewall configurations, determining what traffic can enter or leave a protected network.
• Network Segmentation: Separate different network zones or user groups and restrict communication between them based on security or business needs.
• Quality of Service (QoS): Prioritize certain types of traffic (e.g., voice or video) to ensure optimal performance.