ARP
The broadcast ARP request and the direct reply, the ARP cache, ARP towards the gateway for an outside destination, gratuitous ARP and reading the cache with ip neigh.
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A few cards from the deck
ARP: what is it?
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The protocol by which a host, knowing the IP address of a machine on its network, obtains its MAC address by asking for it in a broadcast.
Sending a frame needs a destination MAC address. The request "Who has 192.168.1.20?" goes out as a broadcast, and the machine concerned answers the sender directly.
What does host 192.168.1.10/24 ask for through ARP before sending a packet to 192.168.1.20, which is on its network?
- The MAC address of 192.168.1.20, in a broadcast
- The gateway's MAC address, in a broadcast
- The IP address of 192.168.1.20, in a broadcast
- The MAC address of 192.168.1.20, by sending it to the gateway
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The MAC address of 192.168.1.20, in a broadcast
The destination is on the same network: delivery is direct, so the MAC address asked for is its own. The IP address is already known; ARP derives the MAC address from it.
The ARP request is sent as a …; the reply goes straight back to the sender, as a unicast.
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The ARP request is sent as a broadcast; the reply goes straight back to the sender, as a unicast.
The request must reach a machine whose MAC address is unknown, so everyone. The reply already knows the asker's address, learned from the request.
ARP cache: what is it?
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The table each host keeps of recently learned IP address to MAC address pairs, forgotten after a delay.
It avoids a broadcast before every packet. Entries age, from a few seconds to a few minutes depending on the system, to keep up with machines that change address.