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Basic Internet Architecture Link layer IP layer Interface 1 Interface 2 Interface 3 ARP Link layer IP layer ARP Link layer IP layer ARP Link layernetwork Figure 4.14 ARP is both peer of, and service for, the IP layer. Both sit over the Link Layer Ethernet frame Interface 1 Interface 2 Destination Source MAC.1 ARP Request Where is 136.80.1.5? EtherType 0 806 Destination MAC.1 Source MAC.2 ARP Reply 136.80.1.5 is at MAC.2 EtherType 0 806 Destination MAC.2 Source MAC.1 IP packet destined for 136.80.1.5 EtherType 0 800 TimeTime Broadcast Unicast Figure 4.15 Frame sequence when initially sending an IP packet from Interface 1 to Interface 2 is ARP for Ethernet [RFC826], usually just referred to as ARP. When an IP address cannot be located in the local interface s ARP table, the interface issues a broadcast ARP Request on the LAN, essentially asking anyone else if they know the mapping. (Transmitting the ARP Request to the Ethernet broadcast address ensures that all attached devices are reached without the local interface needing to know who is, or is not, attached to the LAN at any given time.) Usually, the target (the interface whose IP address is being queried) will respond with a unicast ARP Reply containing the requested IP to Ethernet address mapping. Figure 4.15 shows the packet exchange that would have occurred in the example of Figure 4.12 if Interface 1 s local ARP table did not have a mapping for 136.80.1.5. (An EtherType 0×806 indicates an ARP Request or Reply, while EtherType of 0 800 indicates an IPv4 packet.) The sequence would be as follows: 1. Interface 1 transmits a broadcast ARP Request for 136.80.1.5. 2. Interface 2 unicasts back an ARP Reply (it knows the Ethernet address of Interface 1 from the initial ARP Request) 3. Interface 1 unicasts the IP packet to Interface 2.

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