Access Control Lists | Cisco CCNA 200-301

Summary

This video from 'Cert Bros' offers a comprehensive guide to Access Control Lists (ACLs), explaining their role in network security. ACLs, used by switches and routers, are rule-based lists that permit or deny traffic based on criteria like source and destination addresses. The video delves into standard, extended, and named ACLs, highlighting their unique features and configurations. It also emphasizes the importance of rule order due to the 'implicit deny' rule at the end of every list, ensuring viewers understand the impact of list structure on network traffic control.

Takeaways

• 🔒 Access Control Lists (ACLs) are rule-based lists used by network devices like switches and routers to identify and control traffic based on various criteria.
• 📋 ACLs can be used for purposes beyond just permitting or denying traffic, including network address translation and quality of service configuration.
• 👉 ACLs consist of ordered rules that determine whether traffic should be allowed or blocked; the order is crucial as the first matching rule will be applied.
• ⚠️ There is an implicit 'deny all' rule at the bottom of every ACL, which applies if no other rule matches the traffic.
• 🔢 Standard ACLs use numbers from 1 to 99 or 1300 to 1999 and only consider the source IP address for traffic filtering.
• 🔎 Extended ACLs, with numbers from 100 to 199 or 2000 to 2699, allow for more granular control by considering source and destination IP addresses, protocol, and port numbers.
• 📝 Named ACLs provide a more human-readable alternative to numbered ACLs by assigning names to standard or extended ACLs for easier management.
• 🌐 A wildcard mask is used in standard ACLs to determine which bits of the IP address must match for the rule to apply.
• 🚫 Extended ACLs include an operator to match port numbers, offering options like 'greater than', 'less than', 'not equal to', 'equal to', and 'range'.
• 👀 The configuration of ACLs requires careful planning to ensure that the rules are ordered correctly to avoid unintended traffic blocking or allowing.
• 📚 Understanding how to read and interpret ACLs is essential for network management, as demonstrated by the examples provided in the script.

Q & A

What is an Access Control List (ACL)?

An Access Control List, also known as ACLs or access lists, are rule-based lists used by switches and routers to identify traffic based on source and destination addresses, and port numbers. They are commonly used to permit or deny traffic and can also be used for network address translation and quality of service.

What is the purpose of an ACL in a network?

The primary purpose of an ACL is to control traffic flow by allowing or denying traffic based on specific criteria such as source and destination IP addresses, port numbers, and protocols. This helps in enhancing security and managing network traffic effectively.

How does the order of rules in an ACL affect traffic processing?

The order of rules in an ACL is crucial because the router or switch processes traffic starting from the top of the list and stops at the first matching rule. Therefore, the placement of rules can determine whether intended traffic is permitted or denied.

What is the 'implicit deny' in an ACL?

The 'implicit deny' is an invisible rule at the bottom of every ACL that automatically denies traffic if no matching rule is found as the traffic is processed down the list.

What are the three types of access lists mentioned in the script?

The three types of access lists mentioned are standard access lists, extended access lists, and named access lists. Each type serves different purposes and has different levels of specificity in identifying traffic.

How do standard access lists differ from extended access lists?

Standard access lists use the source IP address only to identify traffic and are identified with numbers between 1 and 99 or 1300 to 1999. In contrast, extended access lists, identified with numbers between 100 and 199 or 2000 to 2699, allow identification based on source and destination addresses, protocol, and port numbers, providing more granular control.

What is the purpose of a wildcard mask in a standard access list?

A wildcard mask in a standard access list works with an IP address to identify which bits of the address need to match the specified IP address. It is like an inverted subnet mask, with zeros indicating bits that must match and ones indicating bits that do not need to match.

What is a named access list and how does it differ from numbered access lists?

A named access list assigns names instead of numbers to standard or extended access lists, making it easier to identify and manage multiple lists on a device. It provides the same functionality as numbered lists but with a more descriptive naming convention.

Can you provide an example of a rule in an extended access list?

An example of a rule in an extended access list could be to deny all TCP traffic from a specific source IP address to a specific destination IP address and port number. For instance, 'deny tcp 192.168.10.0 0.0.0.255 192.168.20.50 21' denies FTP traffic to port 21 on the destination IP 192.168.20.50.

How can the order of rules in an ACL impact the network traffic?

The order of rules in an ACL impacts network traffic because the router or switch stops processing the list at the first matching rule. If a 'permit' rule is placed before a more specific 'deny' rule, the 'deny' rule may never be reached, thus allowing traffic that was intended to be denied.