Which type of ARP message is sent without a request?

Gratuitous ARP is a type of ARP message that is sent without a prior request. This message is typically generated by a host to inform the local network about its own IP-to-MAC address mapping. When a device sends a gratuitous ARP, it broadcasts its own MAC address to the network while claiming an IP address that it has taken ownership of or reconfigured.

This serves several purposes: it helps other devices on the network update their ARP tables with the correct MAC address for a specific IP address, which is particularly useful in scenarios such as IP address conflicts, or when a device moves to a new network segment. Unlike a traditional ARP request, which is sent to inquire about the MAC address associated with a specified IP address, a gratuitous ARP does not wait for a response and actively announces the sender's own address information. This behavior is why it distinctly stands out from other types of ARP messages.