Which characteristic is true regarding standard IPv4 Access Control Lists (ACLs)?

Standard IPv4 Access Control Lists (ACLs) primarily filter traffic based on source IP addresses. This means that when a standard ACL is configured, it assesses the source address of incoming packets to determine whether to permit or deny the traffic. The focus of standard ACLs on source addresses makes them particularly effective for controlling access based on who is attempting to communicate, rather than the specifics of the communication itself, like the destination or the port numbers used.

In contrast, extended ACLs provide more granular control, as they can filter traffic based on both source and destination IP addresses, as well as protocol types and port numbers. This is a key distinction when designing network security and traffic management strategies. The default behavior of standard ACLs, which is to deny traffic that is not explicitly permitted, does not apply here since the question specifically asks about their filtering characteristic.

Understanding this focus on source IP addresses helps in designing ACLs for effective network security measures. Standard ACLs are suitable for scenarios where you only need to determine access based on where the traffic is coming from.