What action can an attacker take to prevent internet access to users?

Substituting an invalid MAC address for the network gateway can effectively disrupt internet access for users on a local network. The media access control (MAC) address is a unique identifier assigned to network interfaces for communications on the physical network segment. By replacing the valid MAC address of the gateway with an invalid one, the attacker can cause devices on the network to be unable to route their traffic to the outside world.

This type of action misleads the devices about where to send their requests for internet access, effectively isolating them from the network gateway that connects to the internet. Without a valid pathway to the gateway, the devices will experience a loss of connectivity, preventing users from accessing online resources.

In contrast to this, encrypting network traffic protects data but does not stop users from accessing the internet. Substituting an invalid IP address for the DNS server would disrupt name resolution but not necessarily prevent internet access altogether, as users could still connect using direct IP addresses. Installing a firewall may control traffic flow and enhance security but is not inherently an action that would prevent internet access to users; rather, it can be configured to allow or deny access based on certain rules.