What is an IP Access Control List (ACL)?
Access Control Lists (ACLs) are one of the most powerful tools in networking for traffic filtering and network security. They allow network engineers to control who can access what in a network by applying sequential rules to packets.
Key Concepts to Remember
Sequential Processing – ACL rules are processed top to bottom
First Match Wins – Once a packet matches a rule, the action is applied immediately
Implicit Deny – Any packet that doesn’t match a rule is denied by default
Directional Application – ACLs can be applied inbound or outbound on interfaces
Types of ACLs
Standard ACL (1–99, 1300–1999)
Filters traffic based only on Source IP address
Extended ACL (100–199, 2000–2699)
Filters based on Source IP, Destination IP, Protocol, and Ports
Where ACLs Are Used
✔ Network Security & Host Protection
✔ Filtering specific traffic (Telnet, FTP, HTTP)
✔ Controlling traffic for NAT and VPN
✔ QoS traffic prioritization
Key Terms
Permit – Allows traffic to pass
Deny – Blocks traffic
Understanding ACLs is essential for CCNA, Network Engineers, and Security Professionals to secure and control network traffic efficiently.
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