Resolve the next hop on the local link
Before sending an IPv4 packet on Ethernet, the host decides whether to send directly to the destination or use a router. If it has no cached MAC address for that next hop, it broadcasts an ARP request for the next hop’s IPv4 address. The owner normally replies to the requester with its MAC address, which the requester can cache.
For an off-link destination, the Ethernet frame goes to the gateway’s MAC address while the IPv4 packet still names the final destination. The host does not broadcast ARP for the remote destination across the router.
Gratuitous, proxy and inverse ARP
Gratuitous ARP is an unsolicited announcement about a sender’s own address. An RFC 5227 ARP Announcement is a broadcast request with the same sender and target IPv4 address. A device can use it to announce that it now owns an address, for example after a gateway moves.
With proxy ARP, a router replies with its own MAC address on behalf of another IPv4 address and forwards the resulting traffic. Inverse ARP starts with a known link identifier and asks for the peer’s network-layer address. Frame Relay uses it to learn the address at the other end of a virtual circuit whose DLCI is already known.
Try it in a lab
- Capture the first ping between two IPv4 hosts on the same VLAN with an empty ARP cache. Identify the broadcast request, the reply and the learned IPv4-to-MAC mapping.
- Then send traffic to an off-link IPv4 host through a router. Compare the destination MAC on the local link with the destination IPv4 address inside the frame.