Ethernet

The target IP and next hop are already known, but packets still cannot be sent. In many cases the problem is not at the IP, TCP, or HTTP layer. It is one step lower: the host knows which IP it wants to reach, but it still does not know which MAC address on the local link should receive the frame.

That is what ARP handles. It looks like a tiny helper protocol, but it actually performs one of the most important handoff tasks in IPv4 local delivery: mapping a Layer-3 address to a Layer-2 reachable object. Without that step, IP knows the direction but not the concrete local receiver.

Two hosts plugged into the same switch do not automatically belong to the same local network. In the field, a problem may look like “the IP is wrong” or “the firewall is blocking it”, but it may have died earlier at the Layer-2 boundary: ARP broadcasts only spread on part of the ports, DHCP Discover never leaves the segment, and packet capture shows traffic on the link while the target device still acts as if it were on another network.

When a host already knows the destination IP, and maybe even the next hop, the packet does not leave the NIC as an IP packet first. What lands on the wire is an Ethernet frame. Problems like “same subnet but no connectivity”, “ARP never gets an answer”, or “DHCP broadcast went out but nobody replied” are not mainly Layer 3 problems. They usually fail earlier, at the Layer 2 hop.