An Access Control List (ACL) is a set of rules that is usually used to filter network traffic. ACLs can be configured on network devices with packet filtering capatibilites, such as routers and firewalls.

ACLs containts a list of conditions that categorize packets and help you determine when to allow or deny network traffic. They are applied on the interface basis to packets leaving or entering an interface. Two types of ACLs are available on a Cisco device:


  • standard access lists – allow you to evaluate only the source IP address of a packet. Standard ACLs are not as powerful as extended access lists, but they are less CPU intensive for the device.
  • extended access lists – allow you to evaluate the source and destination IP addresses, the type of Layer 3 protocol, source and destination port, and other parameters. Extended ACLs are more complex to configure and require more CPU time than the standard ACLs, but they allow more granular level of control.
  • Mandatory access control is a very strict model that was designed for the government. While it is very secure, it can be vague, difficult, and costly. Most organizations rely on mandatory access control in conjunction with one of the other four types.
  • Discretionary access control allows individuals users to decide who can access his or her data. It is often used in social networks when people want to change the visibility of their content. While it is more flexible than mandatory access control, it makes it easy for users to give the wrong people access by accident.
  • Role-based access control allows companies to grant access based on users’ job functions. It is commonly used by businesses to share data with certain departments.
  • Rule-based access control grants or denies access based on pre-defined rules created by an administrator. Users can’t change anything. 
  • Attribute based access control introduces special policies that combine attributes for resources, objects, and users. These may include names, departments, positions, and IP addresses, among others. 


Why Are Access Control Lists Necessary?

Access control lists in networking offer privacy, security, and simplicity for large corporations that house large amounts of data. Below are some additional reasons why a company might use access control lists: 

One of the most important functions of access control lists is the ability to prevent unauthorized users from accessing sensitive services or information. While it is important for employees to be able to access the data they need, it is sometimes even more crucial that a company protects its data from outside individuals. A common example is medical institutions. Hospitals and other health-related facilities need to keep patients’ information private and secure; access control lists are a great way for them to do so. 

Corporations that do business with outside or third-party clients may find access control lists useful because they limit clients’ access to a corporation’s data. This prevents outside individuals from finding sensitive or restricted information. 

Large companies have powerful networks, but even the most intricate networks can only handle so much traffic at once. Networks that receive too much user traffic may slow down, which makes it harder for companies to do business. By controlling how many users can access certain files or systems, access control lists limit network traffic and in turn increase network performance. This saves companies money because they can get the most out of their current network instead of spending to upgrade and increase their network regularly. 

In short, access control lists are an additional form of security that companies can use to safeguard their information. In an age where more people are growing increasingly concerned about the privacy of their data, these benefits can prove invaluable. 

To understand the benefits of using ACLs in your network, consider the following network topology:

acls example

Let’s say that Server holds some important documents that need to be available only to the administrator. We can configure an access list on R1 to enable access to Server only for the administrator’s workstation. Any other traffic going to Server will be blocked. This way, we can ensure that only authorized user can access sensitive files on Server. ACLs are not used exclusively for packet filtering. They are also used to match packets for Network Address Translation (NAT), to match packets to make quality of service (QoS) decisions, and other purposes.

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Author: refuge_2020

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