What is ACL in Networking? Full Form, types & Uses

ACL full form in networking is Access Control Lists. It is a really useful tool to control data flows within a network and keep the data flow protected. ACLs are the most common traffic filtering practices used in routers, switches, and firewalls over IP addresses, protocols, and port numbers.

In this article, we will learn about the Network Access Control List (ACL) and how it works. The article also covers information about the types of ACL, its benefits, and its implementation on Cisco Devices.

Further, if you are an aspiring network professional, you can check out our networking courses to learn more about such technologies.

What is an Access Control List (ACL)? 

Access Control List Definition: It is a set of rules that specifies which users or systems are granted or denied access to particular resources, such as files, directories, or network devices. 

Network Access Control List (ACL) is an important security tool in network management that helps administrators control the flow of data within their network.

We can set specific rules to permit or deny traffic based on factors like IP address, protocol type, and port number. ACLs play a critical role in protecting networks from unauthorized access, improving both security and network performance.

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History of Access Control List 

Initially, ACLs functioned similarly to firewalls by blocking unwanted entities and controlling access to resources. The first implementation of ACLs occurred in 1965 to protect the Multics filesystem.

Over the years, ACLs evolved alongside other access control mechanisms, such as Role-Based Access Control (RBAC) and Attribute-Based Access Control (ABAC).

While many modern firewalls now include network access control features, standalone ACLs are still relevant, especially in conjunction with Virtual Private Networks (VPNs).

Using ACL with VPN allows administrators to define which types of traffic should be encrypted and sent through secure VPN tunnels, enhancing data security during transmission.

Components of an Access Control List

An Access Control List consists of several critical components: 

Types of Access Control List

There are two primary types of Access Control Lists in networking: 

1. Filesystem ACLs 

Filesystem ACLs act as filters for managing access to files and directories. They inform the operating system about which users are permitted to access specific system objects and the privileges associated with those users. 

2. Networking ACLs

Networking ACLs manage access to the network itself. They provide instructions to switches and routers, specifying which types of traffic are permitted to enter the network. They also outline what each user or device can do once they are granted access. 

How Access Control List Works 

Different types of ACLs work in different ways. Let's look at the working of each ACL.

Filesystem ACL Working

In a filesystem context, each file or directory has an associated Access Control List (ACL) that specifies which users or groups can read, write, or execute that file or directory. Here's how Filesystem ACL works:

1. The ACL defines the permissions for each user or group. This granularity allows administrators to specify who can do what with each file or directory.

2. When a user attempts to access a file, whether to read, modify, or execute it, the operating system checks the ACL associated with that file.

3. The operating system assesses the user's identity against the entries in the ACL. If the user's permissions align with what the ACL specifies, access is granted; otherwise, the request is blocked.

Networking ACL Working

Networking ACLs work in a slightly different context, focusing on controlling network traffic. Here's how Networking ACL works:

1. Network administrators define ACLs composed of rules that dictate which types of traffic are allowed or denied.

2. When a data packet arrives at a router or switch, the device inspects the packet against the ACL rules.

3. The device processes the packet according to the rules. If the packet matches a rule that permits access, it is allowed through; otherwise, the packet is discarded.

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Access Control List Rules

ACL rules are the rules that specify whether certain network traffic should be allowed or blocked, based on criteria like IP addresses, protocols, or ports.

Here are some charactersticks of ACL Rules:

● ACL rules are checked in order, from top to bottom. The first matching rule decides if the traffic is allowed or denied.

● If no rule matches, a default “deny all” rule at the end blocks the traffic.

● Rules can permit or deny access based on IP addresses, protocols, and port numbers.

● Overlapping rules are resolved by whichever rule comes first in the list.

● Many systems let you log denied traffic for monitoring and troubleshooting.

● Some ACLs can update rules dynamically based on real-time conditions or user authentication

Benefits of Access Control List

Implementing ACLs offers several advantages: 

● It block unauthorized users.

● It help streamline data flow.

● Administrators can define specific permissions for users based on roles.

● Make it easier to identify and manage users.

● ACLs can be easily adjusted to accommodate organizational changes.

Placement of Access Control List

Network administrators often position ACLs on the edge routers of a network. This strategy allows for traffic filtering before it reaches the core of the system.

For instance, placing an ACL on a routing device between the demilitarized zone (DMZ) and the Internet helps safeguard internal systems. 

Moreover, ACLs can be implemented between the DMZ and the internal network, with each configuration tailored to protect the connected devices and users. 

How to Configure Access Control List?

 To implement the Access Control List on your router, follow the given steps:

Step 1: Log in to Your Router

Open a web browser and enter your router's IP address to access the configuration page. For advanced routers, you may need to use a command line interface.

Step 2: Create Your ACL Rules

Decide which types of traffic you want to allow or block. Consider:

● IP Addresses: Specify where the traffic is coming from and going to.

● Protocols: Choose whether the rule applies to types like TCP or UDP.

● Ports: If needed, indicate specific ports (like port 80 for web traffic).

Step 3: Action

Decide if each rule will permit or deny the traffic.

Step 4: Apply the ACL

Once your rules are set, apply them to a specific part of the router (like an interface) to filter incoming or outgoing traffic.

Step 5: Test Your Settings

Check if your rules are working by trying to access the resources you’ve allowed or blocked. Adjust if necessary.

Step 6: Save Your Changes

Make sure to save your configuration so it stays in place even after the router restarts.

Best Practices for Access Control List Configuration 

● Only allow the traffic that is essential for network operations and deny all else by default. 

● Label your ACLs and rules, if possible, for better organization. 

● Periodically review your ACLs to ensure they align with your current security requirements. 

By following these steps, you can implement effective ACLs on your router, enhancing network security by precisely controlling incoming and outgoing traffic.

Conclusion 

Network Access Control Lists (ACLs) play a vital role in securing network environments by regulating access and filtering traffic.

By implementing both filesystem and networking ACLs, organizations can ensure that only authorized users and devices can interact with their systems, thereby maintaining a robust security posture.

As networks continue to evolve, understanding and effectively managing ACLs will remain crucial for IT administrators.