Cisco Certified Network Associate (CCNA) Practice Exam

What action is necessary for a host with an empty ARP cache to communicate with another host?

1. Send an ARP request for the MAC address of the destination host

2. The default gateway sends an ARP request for the host address

3. Send an ARP request for its own IP address

4. Request an IP address from the DHCP server

The correct answer is: Request an IP address from the DHCP server

When a host has an empty ARP (Address Resolution Protocol) cache, it does not have the MAC address corresponding to any IP address, including that of its intended communication partner. In order to communicate with another host on the local network, the initial action necessary is for the host to send an ARP request to discover the MAC address associated with the destination host's IP address. The correct answer involves sending an ARP request for the MAC address of the destination host. This is fundamental to how ARP operates: when a device needs to communicate with another device, it must map the IP address of that destination to its corresponding MAC address. By sending out an ARP request, it essentially broadcasts a packet into the local network, and the device that has that IP address will respond with its MAC address. This exchange allows the originating host to update its ARP cache and establish a layer 2 (data link layer) connection necessary for sending data frames. Without this ARP request, the host would be unable to send frames to the destination, as it wouldn't know where to send the data on the Ethernet level. Thus, discovering the MAC address with an ARP request is an essential step in the communication process when the ARP cache is empty.