An Access Control List (ACL) is a set of rules or conditions that are used to filter and control the traffic that is allowed or denied through a network device, such as a router or a firewall. ACLs are employed to enhance network security by specifying which packets are permitted and which are denied based on certain criteria. They are commonly used in routers, switches, and firewalls to control access to network resources.

Key Characteristics of Access Control Lists (ACLs):

• Rule-Based Configuration:
  • ACLs consist of rules that define conditions for allowing or denying traffic. Each rule typically includes criteria such as source IP address, destination IP address, protocol, and port numbers.
• Traffic Filtering:
  • ACLs are primarily used for traffic filtering. They can be configured to permit or deny traffic based on specific attributes, such as source and destination addresses, port numbers, or protocols.
• Direction of Traffic:
  • ACLs can be applied to inbound or outbound traffic on an interface. Inbound ACLs filter traffic as it enters an interface, while outbound ACLs filter traffic as it exits.
• Types of ACLs:
  • Standard ACLs: These ACLs filter traffic based on source IP addresses only. They are simpler but less flexible than extended ACLs.
  • Extended ACLs: These ACLs filter traffic based on a variety of criteria, including source and destination IP addresses, protocols, and port numbers.
• Implicit Deny:
  • By default, if a packet does not match any rule in the ACL, it is implicitly denied. This means that ACLs are typically configured to explicitly permit specific traffic and implicitly deny all other traffic.

Configuration Examples:

Standard ACL Example:

Router(config)# access-list 10 permit 192.168.1.0 0.0.0.255

Router(config)# access-list 10 deny any

Router(config)# interface GigabitEthernet0/0

Router(config-if)# ip access-group 10 in

In this example, standard ACL 10 permits traffic from the 192.168.1.0/24 network and denies all other traffic.