Updated on Mar 28, 2024 by

What Is an ACL?

An Access Control List (ACL) comprises individual rules, each representing a decision statement defining packet matching conditions. These conditions can include source addresses, destination addresses, or port numbers. Essentially, an ACL functions as a rule-driven packet filter. It processes packets by adhering to the policies specified within the ACL for those that match the defined criteria.

ACL Composition

ACL rules are designed to either permit or deny specific traffic, necessitating a grasp of the fundamental components of an ACL:

    • ACL ID: shows the number or name used to identify an ACL.

Numeric Identification: Different types of ACLs are distinguished by unique numbers, as per ACL classification.

Named Identification: Alternatively, a string of characters can identify an ACL, offering a more memorable and user-friendly option, akin to using a domain name instead of an IP address.

    • Rule: shows a judgment statement describing a matching condition.

Rule Number: Each rule is assigned a unique number, with rules ordered in ascending sequence.

Action: Specifies whether the device should accept (permit) or discard (deny) packets that match the rule.

Matching Condition: ACLs support diverse matching conditions, encompassing effective time range, IP protocol (e.g., ICMP, TCP, UDP), source/destination IP address, and corresponding port numbers (e.g., 21, 23, 80). Refer to ACL Matching Conditions for more details.

Why Is an ACL Used?

An Access Control List (ACL) acts as a crucial filter, enabling devices to manage and control specific incoming and outgoing traffic. Without the use of ACLs, the network is left vulnerable to potential attacks.

In the following scenario, an enterprise employs an ACL on its router to safeguard financial data. This involves restricting access for R&D department hosts from reaching the financial server while permitting access for hosts in the president's office. Additionally, the ACL helps thwart potential network viruses by blocking commonly used ports, thus fortifying the intranet against malicious intrusions from the Internet.

Key Functions of ACL:

What can an ACL do?

Key Functions of ACL:

    • Secure Access:

ACLs prevent unauthorized access to critical servers, networks, and services, ensuring that enterprise confidential information remains protected.

    • Network Attack Prevention:

By blocking high-risk ports, ACLs serve as a defense mechanism against Internet viruses attempting to infiltrate enterprise intranets.

    • Optimizing Bandwidth Utilization:

ACLs contribute to efficient network bandwidth management, accurately identifying and controlling traffic. This ensures that essential services, particularly those with high quality requirements like voice and video services, receive prioritized bandwidth, enhancing overall user experience.

How Is an ACL Used?


    1. Configure ACL rules:

When setting up ACL rules, it's crucial to identify incoming and outgoing traffic. In this context, incoming traffic enters a device interface (e.g., a router), irrespective of its origin from the Internet or intranet. Similarly, outgoing traffic exits the device interface.

 Incoming traffic and outgoing traffic

Incoming traffic and outgoing traffic

Incoming Traffic: -Traffic entering the router's interface B, whether from the Internet or intranet, carries a public IP address from the Internet.

Outgoing Traffic: -Traffic leaving the router's interface A, whether from intranet to Internet, has an IP address from the intranet as its source.

    1. Apply ACL rules:

To implement ACL rules, apply them to the designated directions (inbound/outbound) of the respective device interfaces. Once configured, apply the ACL rules to the device interfaces to ensure their effectiveness. As ACL-based routing decisions are executed by device hardware, they offer swift and efficient processing.


The device ceases the packet matching process against ACL rules upon finding a match with any rule. It subsequently decides whether to permit or deny the packet based on the matched rule. In the absence of a match with any ACL rule, the device continues to assess the packet against the subsequent rules in the ACL until reaching the end. Typically, an implicit deny statement exists at the ACL's conclusion. Consequently, if a packet fails to match any rule, the device automatically discards it.

ACL Application Scenarios

    • NAT and Intranet Access:

Network Address Translation (NAT) facilitates external user access to the intranet. To bolster intranet security, ACL rules can be configured and applied on the enterprise router. This ensures that only specified external users have access privileges to the intranet.

    • Firewall Protection:

Firewalls, positioned at the intranet and external network edge, play a crucial role in fending off external network attacks and safeguarding significant resources. ACLs configured on the forwarding hardware of devices, particularly firewalls, don't compromise server performance while effectively fortifying network security.

    • QoS-Enhanced Communication Restrictions:

Unrestricted communication across diverse network segments poses security threats. To curb users' access to network segments they don't belong to, an ACL can be applied within a Quality of Service (QoS) traffic policy. This ensures controlled and secure communication between users within specific network segments.

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