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Every Thing You Should Know About ACL

Author Leslie

Date 05/17/2024

This article tells you what ACL is and its principles, so that you can understand its role in the network and how to use it to help enhance your network security.

ACL is a list of rules specifying whether a user can access specific objects or system resources. It is an important part of network security. It provides a protection mechanism to control access to resources based on established rules. Understanding the principles of the access control list is crucial to network security.

What Does ACL Meaning

An access control list is an important tool for managing network access. It is a collection of one or more rules to help protect and manage data traffic. It is widely used in network devices such as routers, switches, and firewalls to authorize or deny access to network traffic by defining rules. This means that data packets must be filtered through the ACL filter when they want to enter or leave the network. If they do not comply with the rules defined by the ACL, they will not be able to pass.

How ACL Works

The working principle of the access control list is that if the network is compared to a company, there will be security personnel at the entrance of the company. The security personnel will hold a list (ACL) in their hands, which stipulates who can enter and who cannot enter. When a data packet wants to enter the network, security personnel will check it one by one according to the access list in their hands. If it meets the defined rules, the data packet will be allowed to enter. But if it doesn't match, the packet won't be able to enter the network. ACL is a checkpoint at the network upload and transmission entrance. The passing data packets are reviewed according to the established rules. If the rules are met, access is allowed, and if they do not, access is denied. It can also control and restrict network access to specific areas, making them critical for the protection of sensitive data. Therefore, choosing an ACL switch can not only limit internal access permissions but also prevent external malicious attacks.

What ACL Does and How To Use It

The Purpose of The Access Control List

ACLs are critical filters used to act as devices such as switches, routers, firewalls, etc. It enables the device to manage the reception and transmission of traffic by allowing or denying requests for data packets. Access control lists can resist potential attacks on the network and protect the security of sensitive data. In enterprises, to protect their sensitive data, they choose to use ACLs on routers to restrict unauthorized traffic from accessing sensitive data. For example, after configuring the access list, you can prevent hosts from other departments of the enterprise from accessing the servers of the financial department, and allow the hosts in the CEO's office to access.

ACL functions

Network traffic filtering: An access list can control the incoming and outgoing network traffic and effectively filter the traffic in and out of network devices by formulating a series of rules that allow or deny access.

Improve network security: Because an access control list can control traffic access and deny unauthorized traffic access and potential attacks, ACL can improve network security.

Optimize network bandwidth: The access list can be combined with the QoS mechanism. When bandwidth is about to be congested, the traffic of certain unnecessary applications can be restricted to ensure data transmission of key applications.

How to Use Access Control List

When it comes to the effective application of ACLs, you need to follow the steps:

Configure ACL rules

You need to determine rules for incoming and outgoing traffic, where incoming traffic refers to data packets entering the device interface, and outgoing traffic values refer to packets exiting the device interface. When an Internet user wants to access the intranet, his traffic will pass through port 2 and carry the public IP address. When an intranet user wants to access the Internet, the traffic will pass through port 1 and carry the IP address of the intranet.

Apply ACLs to critical ports of each device

After setting the access list rules, the key interfaces that need to apply the access control list to each device have been activated. All execution of the hardware is based on its routing and forwarding decisions, so ACL statements can ensure fast execution.

ACL matching mechanism

The matching mechanism between devices and ACLs is as shown below:

When evaluating packets, the device checks them one by one according to the ACL rules. Once a rule matches a packet, the device stops further matching and decides whether to authorize or block access to the packet based on that rule. If the data packet does not match any of the current rules, the device will continue to check the next rule until a matching rule is found or all rules have been checked. Typically, there is a default implicit deny rule at the end of the ACL. Therefore, if a packet fails to match any rules, the device automatically rejects the packet and drops it.

Advantages and Importance Of ACL

Advantages Of Access Control List

Using ACL is crucial for network security, but in addition to network security, an access list has the following advantages:

Enhance network security: ACL can be configured based on multiple conditions such as source IP address, destination IP address, port number, and protocol type, which can accurately control which traffic can enter and prevent unauthorized access. It can also be used to block known malicious IP addresses and ports, reducing the risk of network attacks.

Improve network performance: Access control lists can filter out unnecessary traffic, reduce the processing load of network devices, and improve overall network performance. In addition, ACL can also control the flow of traffic to ensure the bandwidth requirements of key applications and avoid network congestion.

Support compliance and auditing: There are many industries with strict compliance requirements, such as HIPAA, etc. ACL can help administrators understand and track what resources employees access, and restrict access to sensitive data to ensure data security and privacy protection.

Importance Of Access Control List

In a network, ACLs are responsible for performing several key functions

Access lists can configure relevant rules to allow users to support secure access only through specific servers, network areas, and services, thereby avoiding unauthorized access and leakage of sensitive information.

ACLs can block known malicious IP addresses and ports from accessing. They can also block unnecessary and vulnerable ports to protect the network from attacks. This helps to enhance network security and reduce the spread of harmful malware.

Access control list can accurately monitor and manage network traffic. When the network traffic is large, it can reduce some unnecessary traffic to ensure the bandwidth requirements of key applications and improve network bandwidth utilization.


As a vital part of protecting network security in the network, ACL can also improve network performance. It provides a reliable solution for controlling access, optimizing your network, and avoiding threats. If you have any questions about ACL, please feel free to please feel free to contact QSFPTEK's CCIE/HCIE engineers at support@qsfptek.com. Our engineers will provide you with comprehensive support.


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