What is an Access Control List (ACL): A Comprehensive Guide

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access control list

An Access Control List (ACL) is a list of rules that specify which users or systems are granted or denied access to a particular object or system resource. 

It is crucial for managing user permissions related to system resources and locations. 

Although ACLs are beneficial for personal computing, they are primarily used in corporate and multi-user environments to enhance network security, control traffic flow, and prevent unauthorized access to resources like database servers and content management systems.

In this tutorial, we will go into the details of ACL. We will discuss the popular types, components and see how to implement them within a network.

Let’s start with an overview of ACLs.

Table Of Contents

  1. What is ACL (Access Control List)?
  2. Functionality of ACLs
  3. Classification of ACLs: Numbered vs. Named
  4. Types of ACLs in Networking
    1. Type #1: Standard ACL
    2. Type #2: Extended ACL
    3. Type #3: Dynamic ACL
    4. Type #4: Reflexive ACL
    5. Type #5: Time-Based ACL
  5. Components of an Access Control List (ACL)
  6. Advantages of Using Access Control Lists (ACLs)
  7. How to Implement Access Control Lists (ACLs) on Routers
    1. Strategic Tips for ACL Implementation
  8. Conclusion
  9. FAQs

What is ACL (Access Control List)?

An ACL is a collection of rules that either permit or restrict access to a network. 

Network devices, such as routers and switches, enforce these rules for both incoming (ingress) and outgoing (egress) traffic, thus managing which traffic is allowed to traverse the network.

One vital aspect of ACLs in network security is their role in firewall configurations. Firewall ACL serves as a fundamental layer of defense in network security architecture. It effectively regulates data transmission across network boundaries by defining rules that dictate which traffic is allowed or denied based on source and destination IP addresses, ports, and protocols.

Functionality of ACLs

In practical operations, an ACL operates like a stateless firewall. 

Unlike stateful firewalls that inspect packet contents, stateless firewalls simply assess whether packets adhere to established security protocols.

Functionality of ACLs

ACLs are essentially rule-based tables located within network interfaces like routers and switches. Implementing an ACL transforms these devices into traffic filters, determining if traffic from specific users or IP addresses can enter the network.

Classification of ACLs: Numbered vs. Named

Users setting up an ACL have the option to designate it as either a numbered or a named list. Numbered and named ACLs offer a different route to identify and manage ACLs on network devices.

  • Standard ACLs are identified by numbers ranging from 1-99 and 1300-1999.
  • Extended ACLs span numbers between 100-199 and 2000-2699.

Named ACLs are often favored over numbered ones as they allow network administrators to convey more about the ACL’s purpose.

For instance, a named extended ACL intended for the Information Security (InfoSec) department could be initiated with a command with a similar syntax:

# access-list extended INFOSEC [rule1] [rule2] [...]

This naming convention helps clarify the ACL’s function compared to a simple numbered list.

Types of ACLs in Networking

From a security perspective, ACLs are an integral component of network security. During operations, they act as a gatekeeper that filters incoming and outgoing traffic based on predefined rules. 

They come in the following forms: 

  • Standard
  • Extended,
  • Dynamic
  • Reflexive
  • Time-based

Each has its own specific functions and configurations to suit diverse network security needs.

Let’s discuss these types to see how they work. 

Type #1: Standard ACL

The standard ACL is the most basic and simplest form, focusing solely on the source IP address to make decisions.

When a packet attempts to enter or leave a network device, the ACL compares the IP data against established rules. If a matching rule allowing the source IP is found, the filter allows the packet to go through. Otherwise, the packet is denied entry.

For instance, consider an ACL (list number 5) that allows access for all hosts within the network:

# access-list 5 permit

The standard syntax to create a standard ACL is as follows:

# access-list [number or name] {permit | deny} {source [source-wildcard] | host hostname | any}

Here, the source-wildcard is an inverse mask, calculated by subtracting 255 from each subnet mask field. In this example, is the inverse mask for a subnet mask.

Type #2: Extended ACL

Extended ACLs are more detailed than standard ACLs. These lists present a rule-based filtering based on a combination of factors, allowing more precise traffic filtering. The factors can include the following elements:

  • Source and destination IP addresses
  • Protocol type (TCP, UDP, ICMP, etc.)
  • TCP or UDP port numbers

For instance, the following extended ACL (number 150) permits all traffic from the network to any destination using HTTP port 80:

# access-list 150 permit tcp any eq www

The syntax for the extended ACL includes options for protocol, source and destination specifications, and potentially other conditions like precedence, type of service (TOS), and log.

The full syntax is:

# access-list [number or name] {deny | permit} [protocol] [source] [source-wildcard] [destination] [destination-wildcard] [additional-conditions]

Type #3: Dynamic ACL

Dynamic ACL, also known as Lock-and-Key security, introduces an extra layer of adaptability in traffic filtering. It utilizes named extended lists that dynamically adjust rules based on user authentication. This format is particularly useful for scenarios where authorized devices might have changing IP addresses due to relocation or dynamic assignment. 

Type #4: Reflexive ACL

Reflexive ACLs, unlike standard ACLs, track session origins to manage traffic flow initiated from within the host network. 

A Reflexive ACL cannot be applied directly to an interface – instead, they’re typically embedded in an extended named ACL and are not suitable for applications that alter port numbers in mid-session, like FTP.

Type #5: Time-Based ACL

Time-based ACLs regulate access based on time. 

These ACLs enable administrators to regulate network access based on predefined timeframes. For instance, controlling employees’ access to the Internet during designated periods is a great application of these ACLs. 

Creating time-based rules involves the time-range command, which sets up absolute and periodic access restrictions. The time-based ACL syntax is as follows:

# time-range [time-range-name] {start-time | end-time | periodic conditions}

As you can see, each ACL type offers unique benefits and applications, making them vital tools for network administrators aiming to secure and efficiently manage network traffic.

types of ACLs in networking

Components of an Access Control List (ACL)

An access control list (ACL) comprises several key components, each serving a distinct role in defining how access is managed within a network. 

Here are the primary elements that constitute an ACL:

  • Sequence Number: This element assigns a unique identifier to each entry within an ACL. You can use these numbers to organize and prioritize rules.
  • Name: This is the descriptive label for an ACL. Admins use the name for easier identification and management.
  • Statement: This is the core component of an ACL where the actual rules are defined. Statements specify whether to permit or deny access to a particular IP address or range.
  • Network Protocol: This component is used to allow or deny access based on specific network protocols such as IP, TCP, UDP, and others.
  • Source or Destination: This defines whether the rule applies to source or destination IP addresses or ranges, helping to fine-tune access controls.

Some router and hardware network interfaces support additional components for ACLs to enhance functionality and management. Some of these additional components include:

  • Logs: These are used to record events related to the ACL. Logs contain a history of access attempts and changes.
  • Remarks: This feature allows administrators to add notes or comments to ACL entries, clarifying their purpose or rationale. In a multi-admin environment, these remarks are critical to maintaining uniformity and avoiding duplications and user errors.
  • Traffic Control Components: More sophisticated ACLs might include settings for managing network traffic based on Type of Service (ToS) or DSCP (Differentiated Services Code Point) priorities, offering granular control over network performance and security.

Advantages of Using Access Control Lists (ACLs)

Access Control Lists (ACLs) are primarily designed to safeguard networks. As such, they are pivotal tools in network security. Beyond security, ACLs offer several benefits related to the management and control of network traffic:

  • Enhanced Access Management: ACLs enable precise control over who can access network resources, allowing administrators to block specific users or types of traffic. This is especially valuable for protecting servers that are accessible from the Internet.
  • Streamlined User and Host Identification: They facilitate a simple identification process for local and remote users and hosts. As a result, ACLs are crucial for streamlining network management.
  • Congestion Management: ACLs contribute to congestion management by controlling traffic flow. As a result, ACL has minimal impact on network efficiency.
  • Optimized Network Performance: By regulating bandwidth and managing traffic, ACLs prevent network overload, ensuring stable and efficient network performance.
  • Protection Against Attacks: ACLs are effective in mitigating risks such as Denial of Service (DoS) attacks and IP spoofing, enhancing the overall security posture of the network.

How to Implement Access Control Lists (ACLs) on Routers

The best way of implementing firewall ACLs effectively is to apply them directly on routers where you can manage the flow of both inbound (ingress) and outbound (egress) network traffic. Understanding these traffic flows is essential for setting up ACLs appropriately.

The following image Illustrates network traffic:

  • Ingress Traffic: This is the traffic that enters the router from external sources.
  • Egress Traffic: This refers to traffic exiting the router to reach external destinations.

Implement Access Control Lists (ACLs) on Routers

For instance, to block all incoming traffic from the Internet, you would configure an ingress rule with as the source address, representing all external IP addresses, and the destination would be your local network’s IP. 

Conversely, to restrict a specific machine within your network from accessing the Internet, you would set an egress rule where the source is the machine’s IP, and the destination is set to

Strategic Tips for ACL Implementation

We recommend the following tips to simplify the process of ACL setup.

  • Preparation: Always create and configure your ACL on a separate platform before implementing it on a live router or switch. Directly configuring an ACL on the device can lead to interruptions due to the default implicit deny-all rule that activates when the list is empty.
  • Inclusion of Permit Statement: Ensure that your ACL includes at least one permit statement to avoid inadvertently blocking all traffic, which can occur due to the implicit deny statement that concludes every ACL.
  • Rule Order: Place more specific rules higher in the ACL to ensure they are processed before more general rules. This prioritization prevents broad rules from inadvertently overriding more specific intentions.
  • Explicit Deny Statements: Although an implicit deny statement is automatically appended to the end of each ACL, adding an explicit deny statement enhances visibility and monitoring, as it will be logged and shown in the packet denial count when you run the show access-list command.
  • Documentation: Use the remark command to add comments within your ACL. These remarks are invaluable for maintaining clear documentation and understanding the purpose and function of each rule, especially in complex network environments.


Implementing Access Control Lists (ACLs) in firewall settings is a strategic approach to enhance network security. Firewall ACLs serve as a critical defense layer, managing both inbound and outbound traffic to prevent unauthorized access while ensuring that legitimate traffic flows smoothly. By understanding and utilizing ingress and egress traffic rules, organizations can safeguard their network infrastructures from potential threats and disruptions.

In today’s digital landscape, where network security is paramount, integrating ACLs into your firewall configurations not only boosts your security posture but also optimizes network performance by efficiently managing traffic loads. For businesses seeking robust solutions in network management, considering high-resource dedicated hosting can provide the enhanced control, security, and performance necessary to meet complex networking demands.


Q. How do firewall ACLs enhance network security?

Firewall ACLs enhance network security by explicitly allowing or blocking incoming and outgoing traffic based on IP addresses, port numbers, and other network protocols. This helps prevent unauthorized access, block malicious traffic, and manage network traffic more efficiently.

Q. Why consider high-resource dedicated hosting for ACL and firewall management?

High-resource dedicated hosting provides the necessary infrastructure and bandwidth to manage complex ACL configurations and firewall settings without compromising performance. It ensures that security measures do not slow down network speeds and that resources are available to handle high volumes of traffic and sophisticated security protocols.

Q. How can businesses choose the right hosting provider for managing ACLs on firewalls?

When choosing a hosting provider for ACL and firewall management, businesses should consider the following:

  • Security Features: Ensure the provider offers comprehensive security measures, including customizable firewall options.
  • Performance: Check if the provider offers dedicated resources that can handle the business’s traffic and security needs.
  • Support: Look for providers that offer 24/7 technical support and security monitoring.
  • Compliance: Ensure the provider complies with relevant industry standards and regulations.

Q. How can RedSwitches help in implementing firewall ACLs?

RedSwitches offers high-resource dedicated hosting solutions that include support for complex network configurations like firewall ACLs. They provide the infrastructure and expertise to ensure that your network is secure and operates efficiently, allowing you to implement robust security measures like ACLs effectively.

Q. What is an ACL in Network Security?

An ACL (Access Control List) in Network Security is a list of rules that define access rights for a particular system resource, such as a file, folder, or network interface.

Q. How can I use an ACL to control access?

You can use an ACL to control access by setting rules that determine which users or systems can access specific resources and what actions they can perform on those resources.

Q. What are the types of access control lists?

There are two main types of access control lists: standard ACLs and extended ACLs. Standard ACLs control access based on the source IP address, while extended ACLs can also consider factors like destination port and protocol.

Q. What are the components of an ACL?

The components of an ACL include access control entries (ACEs), which define the permissions granted or denied for a specific entity to access a particular resource.

Q. How do ACLs work in network security?

ACLs work by evaluating incoming or outgoing traffic against the rules specified in the list. If a match is found, the corresponding action (allow or deny) is taken to control the traffic flow.

Q. How can I implement an ACL to secure my network?

You can implement an ACL by configuring the rules on your network devices, such as routers or switches, to regulate traffic flow and enhance network security.

Q. What is network access control and why is it important?

Network access control refers to the practice of managing and restricting access to a network based on defined security policies. It is important to prevent unauthorized access and protect sensitive information.

Q. What are extended ACLs and how are they different from standard ACLs?

Extended ACLs provide more granular control over network traffic by allowing additional criteria like destination port, protocol, or specific types of traffic to be considered, whereas standard ACLs are limited to basic source IP filtering.

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