Which message is sent by a host to request the link-layer address of another host?

The message sent by a host to request the link-layer address of another host is the Neighbor Solicitation message. This message is part of the Neighbor Discovery Protocol (NDP) used in IPv6 networks. When a host needs to determine the link-layer address (like a MAC address) of another host on the same local network, it sends out a Neighbor Solicitation message. This message includes the IPv6 address of the target host, allowing recipients to respond with their link-layer address in a Neighbor Advertisement message.

This process ensures that hosts can communicate effectively on the same local link by resolving layer 3 addresses to layer 2, which is essential for the data link layer communication. Understanding this mechanism is crucial for troubleshooting and configuring IPv6 networks, as it replaces the Address Resolution Protocol (ARP) used in IPv4 networks.