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.