Finding a local next hop
Routing first determines the next hop and outgoing link; ARP then obtains the corresponding link-layer address. For a remote destination, this usually means the local router rather than the remote host’s hardware address.
A typical Ethernet ARP request is broadcast locally and answered with a mapping. Hosts retain translation-table entries instead of broadcasting the same question for every packet.
References: [1]
Detecting address conflicts
RFC 5227 defines probes for an IPv4 address a host intends to use. A probe uses zero as the sender IP and the candidate as target, avoiding an assertion that the unconfirmed address is already owned.
An announcement instead states current use. Detecting a conflict requires subsequent handling; a configuration reply alone does not establish that duplicate addressing is impossible.
References: [2]
Mapping, authentication and versions
ARP supplies mappings rather than cryptographic authentication. Conflict detection addresses its own problem, not universal trust in received declarations.
IPv6 uses Neighbor Discovery for local address resolution, router discovery and reachability tasks. Related responsibilities do not make ARP packet formats the standard IPv6 mechanism.