Which protocol is responsible for learning an IPv4 neighbor's MAC address?

The Address Resolution Protocol (ARP) is responsible for mapping an IPv4 address to a machine's MAC address. ARP operates at Layer 2 of the OSI model and is used to find the MAC address of a host given its IPv4 address. When a device wants to communicate with another device on the same local network, it uses ARP to discover the recipient's MAC address.

References:

Juniper official documentation: ARP.

Networking standards: RFC 826.