The Quick Definition: Access lists, also known as access control lists, are configured on routers and used to regulate traffic entering and exiting networks.ACLs are critical components of network security, enabling network administrators to control which devices can communicate with others and under what conditions. These lists can be applied to all routed network protocols, such as IP, and can be configured to filter traffic based on criteria like source IP address, destination IP address, port numbers, and even protocol types. Whether it’s blocking malicious traffic or optimizing network traffic, ACLs are vital for managing and securing a network. Access lists can be configured for all routed network protocols such as IP. Proper configuration of access lists is crucial for anyone preparing for a Cisco certification, as it’s an essential skill tested in Cisco exams. . Proper configuration of access lists is an essential skill for anyone pursuing a Cisco certification, particularly for exams like the CCNA or CCNP. These certifications test knowledge and hands-on experience with configuring, managing, and troubleshooting ACLs on Cisco routers and switches. Understanding how to create and apply ACLs effectively can significantly impact network security and performance. By mastering ACLs, you not only gain valuable skills for Cisco exams but also learn how to secure and optimize real-world networks, which is crucial for advancing in the field of network administration.
Given the importance of ACLs in both exam settings and professional network environments, gaining practical experience in configuring these lists will help ensure success in your Cisco certification journey. Through a hands-on approach to network security, you’ll be prepared to face a variety of scenarios involving traffic control, access restrictions, and security policies.
What is an extended access list?
Extended Access Control Lists (ACLs) act as gatekeepers for your network. They permit or deny traffic based on several parameters, including protocol, port number, source, destination, and even time range. The degree of customization available with ACLs is extensive, making them an essential tool in managing network traffic. For Cisco exam preparation, understanding how to configure ACLs is necessary, as it’s a frequently tested topic. In this example, you’ll learn how to use ACLs to block a specific source from accessing a targeted computer via specific ports. An extended ACL provides a greater degree of control compared to a standard ACL, which typically only filters traffic based on the source IP address. Extended ACLs, on the other hand, can evaluate traffic based on much more granular criteria. For example, they can inspect the protocol type (such as TCP, UDP, or ICMP), the port numbers (like HTTP on port 80 or HTTPS on port 443), and even the direction of the traffic (inbound or outbound). This level of granularity makes extended ACLs ideal for more complex network environments where there is a need to enforce detailed security policies or ensure specific traffic flows.
Extended ACLs can also be applied on Cisco routers to filter traffic passing between different network segments, adding an additional layer of security and performance optimization. By specifying both source and destination IP addresses along with other parameters such as the type of protocol and the port number, extended ACLs can prevent unauthorized access, reduce potential attack vectors, and control the flow of traffic based on security needs. This level of customization is especially important when protecting sensitive network resources or managing traffic between different departments or areas of the network.
For Cisco exam preparation, understanding how to configure and apply extended ACLs is crucial. Many Cisco certification exams, such as the CCNA or CCNP, test your knowledge of ACLs, particularly extended ACLs, as they are a key aspect of network security and traffic management. The ability to configure ACLs properly is often assessed through practical scenarios where you need to create access lists to block or allow specific traffic. Given their importance, learning how to use extended ACLs is not only essential for passing Cisco exams, but also for real-world networking tasks.
Let’s consider a scenario to illustrate their use: imagine you have a network where a particular host, 192.168.1.50, is attempting to access a sensitive server at 192.168.2.50. Normally, you would want to block all traffic between these two IP addresses to prevent unauthorized access. However, using extended ACLs, you don’t need to block all traffic. Instead, you can specifically block access only on ports like HTTP (port 80) and HTTPS (port 443), while allowing other types of traffic to continue.
This example shows how extended ACLs allow for very specific rules to be created that can protect sensitive systems without disrupting normal network operations. This fine level of control is what makes extended ACLs an indispensable tool in Cisco network configuration and security. By mastering the application of extended ACLs, you gain the ability to manage network traffic effectively, a skill that is highly valued in both Cisco practice tests and real-world network management tasks. Understanding ACLs, their application, and their configuration will help you in your journey toward achieving Cisco certifications and succeeding in Cisco exams.
How to Set Up an ACL: Let’s say you have a computer (192.168.1.50) trying to access 192.168.2.50 through HTTP and HTTPS, and you want to prevent that. However, you don’t want to block all traffic between the two IP addresses, as that could cause a network outage. Instead, you want to restrict only HTTP (port 80) and HTTPS (port 443) traffic.However, you don’t want to block all traffic between the two IP addresses entirely, as this could cause a network outage or disrupt necessary communications for other services. Instead, you want to block specific traffic on the HTTP and HTTPS ports, without affecting other types of communication.
The first step is to define the source IP address (in this case, 192.168.1.50) and block traffic from that address. You can use a wildcard mask as a filter within the source group. You need to specify this source IP address in the ACL configuration, and this can be done using a wildcard mask, which acts as a filter to determine which part of the IP address should be evaluated. The wildcard mask essentially tells the router which bits of the IP address should be matched, allowing you to configure specific blocking rules.he wildcard mask allows you to configure the ACL to match exactly the IP address, so only traffic originating from 192.168.1.50 will be affected. You could also use the “host” keyword instead of the wildcard mask in this situation. When you use the keyword “host” before the IP address, it tells the ACL to target exactly that individual IP address, and no mask is needed.
At this point, you’ve defined the source IP address. Now, the next part of the configuration is to set up the destination. In this case, the target server is 192.168.2.50, which is where you want to restrict the HTTP and HTTPS traffic. To specifically block traffic to this destination only on ports 80 and 443, you need to define rules for both ports. This ensures that only web and secure web traffic (HTTP/HTTPS) is denied, while other types of traffic between the source and destination, such as file transfers, email, or ping requests, remain unaffected.
For the HTTP traffic (port 80), the configuration might look like this:
Router1(config)# access-list 150 deny tcp host 192.168.1.50 host 192.168.2.50 eq 80
This line specifically denies TCP traffic from 192.168.1.50 to 192.168.2.50 on port 80 (HTTP). Similarly, for HTTPS traffic on port 443, the command would be:
Router1(config)# access-list 150 deny tcp host 192.168.1.50 host 192.168.2.50 eq 443
Now you have defined the necessary access control rules to block web traffic from the unauthorized source to the destination on the specified ports. Importantly, other traffic, like FTP or ICMP (ping), is not impacted, as the ACL is set to block only traffic matching the defined criteria for ports 80 and 443.
However, an important step that often gets overlooked is the “implicit deny” statement. By default, after all the specified permit or deny statements, Cisco ACLs implicitly deny all traffic that does not match any of the defined rules. Therefore, after you set up your deny statements for HTTP and HTTPS traffic, you need to include a permit statement at the end of your ACL to ensure that all other traffic between the two devices can continue without being blocked.
This permit statement would look like:
Router1(config)# access-list 150 permit ip any any
This command ensures that any IP traffic not explicitly denied will be permitted, allowing other network services to function properly while blocking only the unwanted traffic.
Once the ACL is configured, it needs to be applied to the appropriate interface on the router, and the direction of the traffic flow (inbound or outbound) must be specified. This ensures the ACL is enforced correctly.
By following these steps, you create a fine-grained access control policy that blocks unwanted traffic on specific ports without affecting other necessary communication. This is an essential skill when preparing for Cisco certifications and exams, as configuring ACLs is a common requirement in real-world networking scenarios and often appears in Cisco practice tests. Being able to configure ACLs accurately will help you gain a deeper understanding of network security and traffic management, which is vital for passing Cisco exams like CCNA or CCNP.
For this scenario, entering a mask of 0.0.0.0 will block the entire IP range of the 192.168.1 subnet. Since you’re blocking traffic from just one host, you can use the keyword “host” without a mask.
Setting the Destination: Once you’ve identified the source IP address and have successfully configured the block for that source, the next step in setting up your Access Control List (ACL) is to define the destination, or target, of the traffic you want to control. In this case, you’re targeting a specific destination IP address, 192.168.2.50, and your goal is to restrict access on specific ports, such as HTTP (port 80) and HTTPS (port 443). The challenge here is to ensure that you’re not denying all traffic between the two devices—192.168.1.50 and 192.168.2.50—but only blocking traffic on the specified ports. The first ACL statement you create will block all TCP traffic between the source IP (192.168.1.50) and the destination IP (192.168.2.50) on port 80 (HTTP). To do this, the ACL command would look like:
Router1(config)# access-list 150 deny tcp host 192.168.1.50 host 192.168.2.50 eq 80
In this example:
- The keyword “deny” indicates that you are blocking traffic.
- “tcp” specifies the protocol being filtered (TCP in this case).
- “host 192.168.1.50” designates the source address.
- “host 192.168.2.50” specifies the destination address.
- The keyword “eq” stands for “equal to” and is used to match the exact port numbers. In this case, port 80, which corresponds to HTTP.
The second ACL statement does the same thing, but for HTTPS traffic on port 443:
Router1(config)# access-list 150 deny tcp host 192.168.1.50 host 192.168.2.50 eq 443
This command blocks any TCP traffic from 192.168.1.50 to 192.168.2.50 that is destined for port 443, which is used for HTTPS connections.
By applying these two statements, you’re specifically denying HTTP and HTTPS traffic between the two devices, while leaving other types of communication (such as FTP or ping requests) unaffected. This level of control is a hallmark of extended ACLs, which allow for more precise filtering than standard ACLs, which typically only look at source IP addresses.
To verify the configuration of the ACL and check that the correct rules have been applied, you can use the following command:
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Router1(config)# do show access-list 150
This command will display the current ACL rules and show you how they’ve been applied. The output should look something like this:
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Extended IP access list 150
10 deny tcp host 192.168.1.50 host 192.168.2.50 eq www
20 deny tcp host 192.168.1.50 host 192.168.2.50 eq 443
This shows the two deny statements you configured for blocking HTTP and HTTPS traffic between the two hosts. It’s also important to remember that ACLs process rules sequentially from top to bottom, so the order of these statements is critical. Once the ACL is applied to the correct interface, traffic from 192.168.1.50 trying to reach 192.168.2.50 on port 80 or 443 will be blocked, while all other traffic will still be allowed.
Once you’ve defined the source and destination restrictions, and reviewed your ACL to ensure it matches your expectations, the next step will be to apply the ACL to the correct interface and specify the direction (inbound or outbound). This is a crucial final step to ensure that the ACL rules are actively filtering traffic.
By following this configuration method, you gain precise control over the traffic between specific hosts and ports, enhancing your network’s security. This type of ACL configuration is critical for ensuring that sensitive traffic is blocked while leaving other network services intact—an important skill for preparing for Cisco exams and certification. The ability to properly configure ACLs is often tested in Cisco practice tests and is essential for anyone pursuing a Cisco certification like the CCNA or CCNP.
Negating the “Deny All”: At the end of every ACL, there is an implicit DENY ALL statement. This means if you don’t add a permit statement, all traffic will be denied by default, causing a network outage. The implicit DENY ALL is a built-in feature of ACLs that automatically denies any traffic not explicitly permitted in the ACL configuration.
This implicit deny behavior is meant to ensure that no unauthorized traffic slips through the cracks if no rules are defined to permit it. However, while this security feature can be beneficial for blocking unintentional access, it can also cause problems if not accounted for. Without a permit statement, applying the ACL could result in unintended network disruptions, as it would block all traffic, including legitimate communications.
For example, if you were to apply your ACL with only deny rules, such as those blocking HTTP (port 80) and HTTPS (port 443) traffic between certain hosts, but failed to include a permit statement, you would effectively block all network traffic on that interface. This could lead to a total network outage where even critical services that should be allowed, like DNS, DHCP, or other HTTP/HTTPS communications from different sources, would be blocked.
To avoid this issue, you must explicitly add a permit statement at the end of your ACL. This statement essentially tells the router to allow all other traffic that doesn’t match the deny conditions. In this case, you would add a statement like:
Router1(config)# access-list 150 permit ip any any
This statement ensures that all other IP traffic, from any source to any destination, will be allowed. By placing this permit rule at the end of your ACL, you effectively “negate” the implicit DENY ALL, ensuring that only the traffic you want to block (in this case, the specific ports) will be restricted, while all other traffic can flow freely.
You can verify the updated ACL configuration using the following command:
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Router1(config)# do show access-list 150
This will display the current list of rules configured for access list 150, and you should see that the permit statement has been added to allow all other traffic. The output might look like this:
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Extended IP access list 150
10 deny tcp host 192.168.1.50 host 192.168.2.50 eq www
20 deny tcp host 192.168.1.50 host 192.168.2.50 eq 443
30 permit ip any any
In this case, the ACL includes:
- A deny rule for port 80 (HTTP) between 192.168.1.50 and 192.168.2.50.
- A deny rule for port 443 (HTTPS) between 192.168.1.50 and 192.168.2.50.
- A permit rule that allows all other IP traffic from any source to any destination.
By including the permit rule, you’re ensuring that only the traffic matching the specified deny conditions is blocked, and all other traffic continues to flow as expected.
This concept of negating the DENY ALL is a fundamental aspect of Access Control List (ACL) configuration and is especially important for Cisco exam preparation. In particular, understanding this implicit deny behavior and how to properly configure permit statements is crucial for achieving success in Cisco exams such as the CCNA or CCNP. Whether you’re configuring ACLs to restrict access to certain services or to enhance security, getting the ACL configuration right is essential to ensuring smooth network operations and minimizing potential disruptions.
In practice, this knowledge will come in handy during Cisco practice tests and when configuring ACLs on Cisco devices in real-world scenarios. Properly handling the permit and deny statements and understanding their impact on network traffic flow are critical skills for anyone working with Cisco routers and network security.
With this configuration, the ACL will apply only the deny statements for the specified ports and allow all other traffic.
Applying the ACL and Determining Direction: Following Cisco best practices, it’s recommended to apply the ACL as early as possible in the sequence, which, in this case, is at Router 1. You can configure the interface and apply the ACL to it using the following commands:
Router1(config)# int fa0/0
Router1(config-if)# ip access-group 150 in
You’ll need to specify the direction (inbound or outbound) of the traffic. In this case, traffic is incoming on the interface, so the final command is in. This ensures that host 192.168.1.50 will no longer be able to access host 192.168.2.50 via HTTP or HTTPS.
Conclusion
By following the above steps, you’ve successfully configured an extended access list (ACL) to block specific traffic based on IP address and port number. This process is critical for anyone pursuing Cisco certification and preparing for Cisco practice tests or exams. Understanding and implementing ACLs is a fundamental skill tested in the Cisco exam, and having a hands-on approach in configuring them will help ensure success in your Cisco certification journey. The process of configuring ACLs not only helps secure a network by regulating traffic but also aids in optimizing network performance. As network environments become more complex, ACLs provide an effective way to enforce security policies and ensure that only authorized traffic is allowed. In addition to blocking traffic based on IP address and ports, ACLs can also be customized further to filter traffic by protocols, time ranges, and even specific users. This level of control is what makes ACLs indispensable in network security.
For those looking to dive deeper into network security and access control configurations, completing a Cisco dumps course or practicing with mock exams can enhance understanding and readiness for real-world networking scenarios.
