Access Control List (ACL) - What are They and How to Configure Them! • ITT Systems (2023)

In the computer networking world, an ACL is one of the most fundamental components of security.

An Access Control Lists “ACL” is a function that watches incoming and outgoing traffic and compares it with a set of defined statements.

In this article, we will go deep into the functionality of ACLs, and answer the following common questions about ACLs?

  1. What is an Access Control List?
  2. Why Use An ACL?
  3. Where Can You Place An ACL?
  4. What Are The Components of An ACL?
  5. What Are The Types of ACLs?
  6. How to Implement An ACL on a Router?

What is an Access Control List?

Access Control Lists “ACLs” are network traffic filters that can control incoming or outgoing traffic.

ACLs work on a set of rules that define how to forward or block a packet at the router’s interface.

An ACL is the same as a Stateless Firewall, which only restricts, blocks, or allows the packets that are flowing from source to destination.

When you define an ACL on a routing device for a specific interface, all the traffic flowing through will be compared with the ACL statement which will either block it or allow it.

The criteria for defining the ACL rules could be the source, the destination, a specific protocol, or more information.

ACLs are common in routers or firewalls, but they can also configure them in any device that runs in the network, from hosts, network devices, servers, etc.

Why Use An ACL?

The main idea of using an ACL is to provide security to your network. Without it, any traffic is either allowed to enter or exit, making it more vulnerable to unwanted and dangerous traffic.

To improve security with an ACL you can, for example, deny specific routing updates or provide traffic flow control.

As shown in the picture below, the routing device has an ACL that is denying access to host C into the Financial network, and at the same time, it is allowing access to host D.

Access Control List (ACL) - What are They and How to Configure Them! • ITT Systems (1)

(Video) What Are Access Lists? -- Access Control Lists (ACLs) -- Part 1 of 8

With an ACL you can filter packets for a single or group of IP address or different protocols, such as TCP or UDP.

So for example, instead of blocking only one host in the engineering team, you can deny access to the entire network and only allow one. Or you can also restrict the access to host C.

If the Engineer from host C, needs to access a web server located in the Financial network, you can only allow port 80, and block everything else.

Where Can You Place An ACL?

The devices that are facing unknown external networks, such as the Internet, need to have a way to filter traffic. So, one of the best places to configure an ACL is on the edge routers.

A routing device with an ACL can be placed facing the Internet and connecting the DMZ (De-Militarized Zone), which is a buffer zone that divides the public Internet and the private network.

The DMZ is reserved for servers that need access from the outside, such as Web Servers, app servers, DNS servers, VPNs, etc.

As shown in the picture below, the design shows a DMZ divided by two devices, one that separates the trusted zone from the DMZ and another that separates it with the Internet (public network).
Access Control List (ACL) - What are They and How to Configure Them! • ITT Systems (2)

The router facing the Internet acts as a gateway for all outside networks. It provides general security by blocking larger subnets from going out or in.

You can also configure an ACL in this router to protect against specific well-known ports (TCP or UDP).

The internal router, located between the DMZ and the Trusted Zone, can be configured with more restrictive rules to protect the internal network. However, this is a great place to choose a stateful firewall over an ACL.

But Why is it Better to place an ACL vs. Stateful Firewall to protect the DMZ?

ACLs are directly configured in a device’s forwarding hardware, so they do not compromise the end performance.

Placing a stateful firewall to protect a DMZ can compromise your network’s performance.

(Video) Access Control Lists | Cisco CCNA 200-301

Choosing an ACL router to protect high-performance assets, such as applications or servers can be a better option. While ACLs might not provide the level of security that a stateful firewall offer, they are optimal for endpoints in the network that need high speed and necessary protection.

What Are The Components of An ACL?

The implementation for ACLs is pretty similar in most routing platforms, all of which have general guidelines for configuring them.

Remember that an ACL is a set of rules or entries. You can have an ACL with single or multiple entries, where each one is supposed to do something, it can be to permit everything or block nothing.

When you define an ACL entry, you’ll need necessary information.

  1. Sequence Number:
    Identify an ACL entry using a number.
  2. ACL Name:
    Define an ACL entry using a name. Instead of using a sequence of numbers, some routers allow a combination of letters and numbers.
  3. Remark:
    Some Routers allow you to add comments into an ACL, which can help you to add detailed descriptions.
  4. Statement:
    Deny or permit a specific source based on address and wildcard mask. Some routing devices, such as Cisco, configure an implicit deny statement at the end of each ACL by default.
  5. Network Protocol:
    Specify whether deny/permit IP, IPX, ICMP, TCP, UDP, NetBIOS, and more.
  6. Source or Destination:
    Define the Source or Destination target as a Single IP, a Address Range (CIDR), or all Addresses.
  7. Log:
    Some devices are capable of keeping logs when ACL matches are found.
  8. Other Criteria:
    Advanced ACLs allow you to use control traffic through the Type of Service (ToS), IP precedence, and differentiated services codepoint (DSCP) priority.

What Are The Types of ACLs?

There are four types of ACLs that you can use for different purposes, these are standard, extended, dynamic, reflexive, and time-based ACLs.

1. Standard ACL

The standard ACL aims to protect a network using only the source address.

It is the most basic type and can be used for simple deployments, but unfortunately, it does not provide strong security. The configuration for a standard ACL on a Cisco router is as follows:

Access Control List (ACL) - What are They and How to Configure Them! • ITT Systems (3)

2. Extended ACL

With the extended ACL, you can also block source and destination for single hosts or entire networks.

You can also use an extended ACL to filter traffic based on protocol information (IP, ICMP, TCP, UDP).

The configuration of an extended ACL in a Cisco router for TCP is as follows:

Access Control List (ACL) - What are They and How to Configure Them! • ITT Systems (4)

3. Dynamic ACL

Dynamic ACLs, rely upon extended ACLs, Telnet, and authentication. This type of ACLs are often referred to as “Lock and Key” and can be used for specific timeframes.

(Video) Applying Access Control Lists (ACLs)

These lists permit access to a user to a source or destination only if the user authenticates to the device via Telnet.

The following is the configuration of a Dynamic ACL in a Cisco router.

Access Control List (ACL) - What are They and How to Configure Them! • ITT Systems (5)

4. Reflexive ACL

Reflexive ACLs are also referred to as IP session ACLs. These type of ACLs, filter traffic based on upper layer session information.

They react to sessions originated inside the router to whether permit outbound traffic or restrict incoming traffic. The router recognizes the outbound ACL traffic and creates a new ACL entry for the inbound.

When the session finishes, the entry is removed.

The configuration of a reflexive ACL in a Cisco router is as follows:

Access Control List (ACL) - What are They and How to Configure Them! • ITT Systems (6)

How to Implement An ACL On your Router?

Understanding ingress and egress traffic (or inbound and outbound) in a router, is critical for proper ACL implementation.

When setting rules for an ACL, all traffic flows are based on the point-of-view of the router’s interface (not the other networks).

As you can see from the picture below, ingress traffic is the flow coming from a network, whether it is external or internal, into the router’s interface. The egress traffic, on the other hand, is the flow from the interface going out into a network.

Access Control List (ACL) - What are They and How to Configure Them! • ITT Systems (7)

For an ACL to work, apply it to a router’s interface. Since all routing and forwarding decisions are made from the router’s hardware, the ACL statements can be executed much faster.

(Video) Understanding Access Control Lists | Network Fundamentals Part 14

When you create an ACL entry, the source address goes first, and the destination goes after. Take the example of the extended ACL configuration for IP on a Cisco Router. When you create a Deny/Permit rule, you must first define the source, and then the destination IP.

Access Control List (ACL) - What are They and How to Configure Them! • ITT Systems (8)

The incoming flow is the source of all hosts or network, and the outgoing is the destination of all hosts and networks.

What is the Source if you want to Block Traffic coming from the Internet?

Remember that inbound traffic is coming from the outside network to your router interface.

So the source is an IP address from the Internet (a web server public IP address) or everything (wildcard mask of, and the destination is an internal IP address.

On the contrary, what if you what to Block a Specific Host to connect to the Internet?

The inbound traffic is coming from the inside network to your router interface and going out to the Internet. So the source is the IP from the internal host, and the destination is the IP address on the Internet.


ACLs are the packet filters of a network.

They can restrict, permit, or deny traffic which is essential for security. An ACL allows you to control the flow of packets for a single or group of IP address or different for protocols, such as TCP, UDP, ICMP, etc.

Placing an ACL on the wrong interface or mistakenly changing source/destination can create a negative impact on the network. A single ACL statement can leave an entire business without the Internet.

To avoid negative performance is critical to understand the inbound and outbound traffic flows, how ACLs work, and where to place them. Remember that a router’s job is to forward traffic through the right interface so that a flow can be either coming it (inbound) or going out (outbound).

Although a stateful firewall provides much better security, they can compromise the performance of the network. But an ACLs is deployed right on the interface, and the router uses its hardware capabilities to process it, making it much faster and still giving a good level of security.

(Video) Router' ACL- Access Control List

Access Control List FAQs

What is in an access control list?

An access control list (ACL) contains rules about access to a service or resource. The grantee can be a user or a system, such as a piece of software. When implemented on a router at the network’s boundary, an ACL acts as a firewall, blocking access from banned addresses and filtering out specific content. An ACL can also be placed on outgoing traffic. ACLs can also be set up on switches to control internal network traffic. ACL rules can be sophisticated and combine source and destination, thus, a user can be blocked from access to certain destinations on the network.

What is a standard ACL?

A standard access control list is one of the four types of ACL. This evaluates the source of each packet, passing through the controlled device. The other three types are extended ACL, which allows filtering based on source, destination, and protocol; dynamic ACL, which looks at source and destination and also requires user authentication via Telnet; and reflexive ACL, which coordinates source and destination address on multiple packets, only allowing a packet in if it is a reply to an outbound packet.

What are different types of ACL in firewall?

When used in firewalls, ACLs can control access to file systems or networks. So, the two types of ACLs are networking ACLs and filesystem ACLs.


What is an access control list ACL and how is an ACL used? ›

Access control lists are used for controlling permissions to a computer system or computer network. They are used to filter traffic in and out of a specific device. Those devices can be network devices that act as network gateways or endpoint devices that users access directly.

What is an access control list what are the types of ACLs? ›

An access control list (ACL) contains rules that grant or deny access to certain digital environments. There are two types of ACLs: Filesystem ACLs━filter access to files and/or directories. Filesystem ACLs tell operating systems which users can access the system, and what privileges the users are allowed.

How are ACL configured? ›

ACL Configuration Guidelines

Once a packet meets the ACL criteria, the ACL processing stops and the packet is either permitted or denied. ACLs are created globally and then applied to interfaces. An ACL in network configuration can filter traffic going through the router, or traffic to and from the router.

What is ACL and how do you configure a new ACL for a site? ›

Access Control Lists (ACLs) are a collection of permit and deny conditions, called rules, that provide security by blocking unauthorized users and allowing authorized users to access specific resources.

What is an ACL and what is its purpose? ›

The anterior cruciate ligament (ACL) is one of the key ligaments that help stabilize the knee joint. The ACL connects the thighbone (femur) to the shinbone (tibia). It's most commonly torn during sports that involve sudden stops and changes in direction — such as basketball, soccer, tennis and volleyball.

How does the ACL work? ›

Normally ACLs reside in a firewall router or in a router connecting two internal networks. When you configure ACLs, you can selectively admit or reject inbound traffic, thereby controlling access to your network or to specific resources on your network. You can set up ACLs to control traffic at Layer 2-, or Layer 3.

What are the 3 types of access control? ›

Three main types of access control systems are: Discretionary Access Control (DAC), Role Based Access Control (RBAC), and Mandatory Access Control (MAC). DAC is a type of access control system that assigns access rights based on rules specified by users.

What are the four 4 main access control model? ›

Currently, there are four primary types of access control models: mandatory access control (MAC), role-based access control (RBAC), discretionary access control (DAC), and rule-based access control (RBAC).

What are the 5 areas of access control? ›

The 5 Different Types of Access Control
  • Manual access control.
  • Mechanical access control.
  • Electronic access systems.
  • Mechatronic access control.
  • Physical access systems.

What are three types of ACL? ›

There are four different types of ACLs, each of which has a different use. they are reflexive, extended, dynamic, and standard.

How many total ACLs can be configured on an interface? ›

Standard ACLs permit or deny packets based on source and destination IP address. You can configure up to 99 standard ACLs. You can configure up to 1024 individual ACL entries on a device.

How do I configure access control list in Windows? ›

How to set ACLs
  1. Click the Add... button. ...
  2. In the Name: field type the SUNet ID of the person you want to add. ...
  3. Click on the r - Read , l - Lookup, i - Insert, d - Delete, w - Write, and k - Lock buttons.
  4. Click the OK button.
  5. Check the Set AFS ACL window to make sure your addition was recorded.
  6. Click OK.
Dec 9, 2015

Why do you need an ACL? ›

It's the job of your ACL to keep your knee stable during quick changes in direction, a necessary function for sports like soccer, basketball, football, and lacrosse. An ACL injury, which can range from a sprain to a full rupture, is not only painful, but can put an abrupt end to an athletic season.

What does the ACL prevent? ›

Anterior cruciate ligament (ACL) is in the middle of the knee. It prevents the shin bone from sliding out in front of the thigh bone. Posterior cruciate ligament (PCL) works with the ACL. It prevents the shin bone from sliding backward under the femur.

How many ACLs do you have? ›

There is a ligament on each side of the knee (the collateral ligaments) and two ligaments deep inside the knee. The two ligaments inside the knee that “cross” each other are called the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL).

What is the difference between ACL and firewall? ›

A firewall has one main use and purpose and that is to examine traffic passing through a part of the network and make decisions about what to let through and what to block. ACLs do stateless inspection, which means that the access list looks at a packet and has no knowledge of what has come before it.

What kind of firewall is an ACL? ›

An ACL works as a stateless firewall. While a stateful firewall examines the contents of network packets, a stateless firewall only checks if the packets follow the defined security rules. ACLs are tables containing access rules found on network interfaces such as routers and switches.

Why are ACLs configured on the distribution layer? ›

The primary reason is to provide a basic level of security for the network. ACLs are not as complex and in depth of protection as stateful firewalls, but they do provide protection on higher speed interfaces where line rate speed is important and firewalls may be restrictive.

What are the six main categories of access control? ›

As noted above, the CISSP exam calls out six flavors of access control.
  • Mandatory Access Control (MAC) ...
  • Discretionary Access Control (DAC) ...
  • Role-Based Access Control (RBAC) ...
  • Rule-Based Access Control. ...
  • Attribute-Based Access Control (ABAC) ...
  • Risk-Based Access Control.

What are two main types of access control? ›

What are the Different Types of Access Control Systems?
  • Discretionary Access Control (DAC) A discretionary access control system, on the other hand, puts a little more control back into the business owner's hands. ...
  • Rule-Based Access Control. ...
  • Identity-Based Access Control.
Jul 21, 2021

What are the basics of access control? ›

Access control identifies users by verifying various login credentials, which can include usernames and passwords, PINs, biometric scans, and security tokens. Many access control systems also include multifactor authentication (MFA), a method that requires multiple authentication methods to verify a user's identity.

How do you implement access control? ›

6 Tips For Implementing Access Control Authentication System With Security
  1. Implement a central repository with well-defined whitelisting policies. ...
  2. Solve self-generated scripts. ...
  3. Withdraw your departing employees' digital rights. ...
  4. Adapt your access control. ...
  5. Create consistent processes to whitelist new cloud applications.
Oct 1, 2018

What are examples of an access control system? ›

These are the most common types of access control:
  • Keypad readers. A keypad door reader requires a user to type in a PIN or passcode to unlock the door. ...
  • Swipe card readers. ...
  • RFID door readers. ...
  • Biometric door readers. ...
  • Smart lock door readers.

What are the four control systems? ›

The four types of control systems are belief systems, boundary systems, diagnostic systems, and interactive system.

What are the six 6 benefits of access control? ›

The most significant benefits of access control systems are:
  • Increase Ease of Access for Employees. ...
  • Get Rid of Traditional Keys. ...
  • Save Money and Energy. ...
  • Keep Track of Who Comes and Goes. ...
  • Protect Against Unwanted Visitors. ...
  • Give Employees the Freedom to Work When They Need To. ...
  • Prevent Against Data Breaches.

What are the 2 most popular tests for the ACL? ›

The anterior Lachman test, anterior drawer test and the pivot shift test, which are summarised in Table 1, are the most commonly known physical tests used to assess the integrity of the ACL (Benjaminse 2006; Leblanc 2015; Malanga 2003; Scholten 2003; Solomon 2001).

What is the structure of the ACL? ›

The ACL has a microstructure of collagen bundles of multiple types (mostly type I) and a matrix made of a network of proteins, glycoproteins, elastic systems, and glycosaminoglycans with multiple functional interactions.

What is ACL in Active directory? ›

An access control list (ACL) is a list of access control entries (ACE). Each ACE in an ACL identifies a trustee and specifies the access rights allowed, denied, or audited for that trustee. The security descriptor for a securable object can contain two types of ACLs: a DACL and a SACL.

Does a router have ACLs configured by default? ›

A router does not have any ACLs configured by default. However, when an ACL is applied to an interface, the router performs the additional task of evaluating all network packets as they pass through the interface to determine whether the packets can be forwarded.

How do I display ACLs on a directory? ›

To display the ACL associated with a file, directory or symbolic link, issue the fs listacl command. The output for a symbolic link displays the ACL that applies to its target file or directory, rather than the ACL on the directory that houses the symbolic link.

What are the 9 cases in ACLs provider manual? ›

  • ACLS Cases Respiratory Arrest.
  • Pulseless Ventricular Tachycardia and Ventricular Fibrillation.
  • Pulseless Electrical Activity (PEA) & Asystole.
  • Adult Cardiac Arrest Management Algorithm.
  • Cardiac Arrest Management.
  • Adult Immediate Post Cardiac Arrest Management Algorithm.
  • Symptomatic Bradycardia-Slow Heart Rate.

What is access control configuration? ›

Such support is built around the OAuth 2.0 protocol. An access control policy is a set of conditions that, after they have been evaluated, determine access decisions. The administrator can specify a custom domain to separate metadata in a registry.

What is an Access Control List ACL quizlet? ›

An ACL or Access control list is a common means by which access to and denial of services is controlled. On network devices such as Routers and firewalls, they act as filters for network traffic, packet storms, services and host access. Most of these devices come with standard or default ACL and allow for custom ACL's.

What is the use of ACL in AWS? ›

Amazon S3 access control lists (ACLs) enable you to manage access to buckets and objects. Each bucket and object has an ACL attached to it as a subresource. It defines which AWS accounts or groups are granted access and the type of access.

What is an Access Control List ACL ServiceNow? ›

ACLs or Access Control Lists are the process by which ServiceNow provides granular security for its data and can be applied to individual records, as well as fields within those records.

What is the use of ACL in Kafka? ›

Access Control Lists (ACLs) provide important authorization controls for your enterprise's Apache Kafka® cluster data.

Which of the following best defines an access control list ACL )? ›

An access control list, abbreviated as ACL, is a list of rules that outline which users or systems are granted or denied access to a particular object or system. The rules describe the packet matching conditions, such as the source address, destination address, and port number of packets.

Is ACL same as firewall? ›

A firewall has one main use and purpose and that is to examine traffic passing through a part of the network and make decisions about what to let through and what to block. ACLs do stateless inspection, which means that the access list looks at a packet and has no knowledge of what has come before it.

Why do we need ACL? ›

You can use an ACL to control packet flow for a single or a group of IP addresses, as well as for different protocols like TCP, UDP, and ICMP, and so on. Using an ACL to restrict access to an unacceptable interface or a source/objective that is erroneously evolving could have a negative impact on the business.

What are 3 types of ACL in Servicenow? ›

  • Example - Restrict a table.
  • Example - Restrict a field.
  • Example - Restrict a field with a script.
  • Example - Restrict a field with a condition.
Jul 29, 2020

How many types of ACL are there in Servicenow? ›

when we talk about record type than acl can be applied on Row level or Field level or in other words we can say two type of record acl's i.e., Row level acl and field level acl (column level) acl.


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