On a local network, a host often knows another machine’s network address and still needs the hardware address that will carry the frame. Address resolution is the request and the reply that fill in that hardware address.
How a Host Learns the Hardware Address
The host asks, on the local segment, which machine owns the network address it already has. The owner answers with its hardware address. The asker stores that pair and uses it for later frames, until the stored answer grows old.
- The ask. One host broadcasts a request that names the network address it wants to reach.
- The answer. The owner replies with the hardware address of its own interface.
- The stored pair. The asker keeps the pair so the next frame does not need a new request.
What the Stored Answer Does Not Prove
The reply says which hardware address answered. It does not prove that the answering machine is the one the host meant to reach. A later reply can replace the stored pair, and the host will use the new hardware address until that reply grows old as well.
| Step | What the Host Already Knows | What the Reply Adds |
| Before the request | The network address it wants to reach | Nothing yet |
| The request | That same network address, sent to the whole segment | Still waiting |
| After the reply | The network address | The hardware address to put on the frame |