Home > Articles > Cisco > CCNA Routing and Switching

Mastering Access Control Lists

  • Print
  • + Share This
In order to achieve success at the CCNA level and beyond, you must master Access Control Lists (ACLs). When most CCNA candidates think about ACLs, they immediately think about filtering traffic with these powerful structures, but filtering is only one of the many uses these structures have for Cisco network engineers. We end up using these structures for classification of traffic for many, many different purposes in Cisco IOS. This article discusses the various types of ACLs, the implicit deny all feature, the importance of order, and assigning ACLs as filters.
Like this article? We recommend

Like this article? We recommend

In order to achieve success at the Cisco Certified Network Associate (CCNA) level and beyond, you must master Access Control Lists (ACLs). There is simply no doubt about this. Part of the reason is that we find we must use ACLs for so many functions these days on our Cisco devices, devices like Routers, Switches, and Firewalls.

When most CCNA candidates think about ACLs, they immediately think about filtering traffic with these powerful structures. Perhaps permitting one form of traffic from the Internet into their private network, but blocking other forms of traffic. But filtering is only one of the many uses these structures have for Cisco network engineers now. We end up using these structures for classification of traffic for many, many different purposes in the Cisco Internetwork Operating System (IOS). For example, perhaps we need to define what addresses will be translated with Network Address Translation (NAT), or perhaps we desire to highlight some traffic for transmission over a Virtual Private Network (VPN). Yes, you guessed it, it is Access Control Lists to the rescue.

Types of Lists

While there are many types of access lists for various protocols, this article will focus on IP Access Control Lists. It probably comes as no surprise to our readers that these are the most popular form of ACLs used today. This is because IP is quickly becoming a ubiquitous protocol in our modern networks.

When it comes to our IP ACLs, the first distinction we need to be aware of is between a named ACL and a numbered ACL. When ACL technology was first invented, it was a numbered structure only. The number that is used to identify the ACL educates the Cisco IOS about what type of ACL it is. For example, an ACL with a number of 10 indicates the ACL is an IP type, and it also indicates that it is a standard ACL.

This leads us to our next distinction of different types of ACLs. With IP access lists, there are standard versus extended ACLs. A standard ACL is only able to distinguish traffic based on the source IP address field in the packet. An extended ACL, however, is able to distinguish traffic based on the exact IP protocol type, the source address, the destination address, even the source and destination port information. Extended ACLs certainly enhance the flexibility of use and the granularity of these lists, but their power is not always needed. In fact, when all we care about is the source IP address in a packet, the standard ACL remains the logical choice.

Here is an example of a numbered ACL:

R1(config)#access-list 10 permit
R1(config)#access-list 10 permit

Notice this ACL consists of two entries. Traffic matching the 10.10.X.X network or the 10.20.X.X network is permitted by this list. Notice the Wildcard Masks that are used to distinguish which portion of the address to match. I like to call Wildcard Masks Inverse Masks. This helps me to remember that they work exactly opposite of how Subnet Masks work. In the Inverse Mask, a zero bit setting indicates we want to match, while the one bit setting indicates “we don’t care.”

Note that numbered ACLs from 1-99 are for standard IP, while numbers from 100-199 are for extended IP lists. There are other “expanded” number ranges set aside, but these should be enough for the average job. And let’s face it...if we run out, we can always use named ACLs.

Here is an example of a named, standard ACL:

R1(config)#ip access-list standard AL_EXAMPLE

Notice how the name is beneficial to communicate the purpose of the ACL. Notice also how we had to specify that it is a standard IP list we are creating since there is no number used to communicate that information.

Here is an example of a numbered, extended ACL:

R1(config)#access-list 102 deny icmp any any echo
R1(config)#access-list 102 permit ip any any

Notice how this list allows us to deny very specific traffic[md]in this case[md]an echo packet used by the PING utility. Notice also how access lists allow us to use the shortcut keyword any to represent

The Implicit Deny All

One of the critical aspects of any ACL that we must remember is that the Access List ends with an implicit DENY ALL statement. Implicit means that we cannot see this Access Control Entry (ACE) when we look at the ACL. We cannot see it[md]but it is there, providing a DENY ACL treatment to any traffic that makes its way down the entire ACL without a matching statement.

Often times engineers will want to know how many packets hit the implicit deny all at the end of a list. Because of this need, they create an explicit deny all entry to conclude the ACL. Here is an example:

R1(config)#access-list 103 permit ip
R1(config)#access-list 103 deny ip any any

The Importance of Order

The order of statements is critical in your ACLs. This is because there is a rigid top-down processing that occurs, and once there is a match for the traffic, no more processing occurs for this packet. As a result, always remember to place your more specific entries earlier in your ACLs. Here is an example:

R1(config)#ip access-list standard AL_ORDERISIMPORTANT
R1(config-std-nacl)#permit any

Notice in this example, we must place the permit for the specific host ( before the more general denying of all traffic in the 10.X.X.X network. If we did not do this, the traffic would be denied before hitting any other permit entry.

Assigning ACLs as Filters

It always tends to strike students by surprise that on an interface, only one ACL per protocol, per direction is permitted. So if you have a Fa0/0 interface and you want to filter inbound IP traffic using an ACL, you can only have one of them. This means that your ACLs can get really long and really complex pretty quickly. You should always plan on saving them in some type of data store outside of the actual Cisco device as a backup, and to give yourself an easy editing location.

What syntax is used to actually assign an ACL filter? Here is an example:

R1(config-if)#ip access-group AL_EXAMPLE in

Notice that we are in interface configuration mode and we use the command ip access-group {name | number} {in | out}.

If you want to filter traffic on the Virtual Terminal Lines into the device, the command to assign the ACL is a bit different, the command is as follows:

R1(config-line)#access-class 100 in

Be sure to practice with the concepts presented in this article to ensure your mastery of these important Cisco IOS constructs.

  • + Share This
  • 🔖 Save To Your Account

Pearson IT Certification Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from Pearson IT Certification and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about Pearson IT Certification products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites; develop new products and services; conduct educational research; and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by Adobe Press. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.pearsonitcertification.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020