Specifications

Installation and Operation Manual Appendix C Operating Environment
FCD-E1LC Ver. 1.0 Ethernet Transmission Technology C-17
ARP Protocol
When sending IP packets over Ethernet, it is necessary to determine the MAC
address of the destination, to insert it in the Ethernet destination MAC address
of the packet. Actually, this is necessary for any physical transmission technology
which is not limited to point-to-point topologies.
This is performed by means of the ARP (Address Resolution Protocol), part of the
IP suite of protocols. ARP is used to generate a look-up table that translates IP
addresses to MAC addresses for any transmission technology. The translation is
done only for outgoing IP packets, because this is when the IP header and the
Ethernet header are created.
The ARP table contains one row for each IP host: each row has two columns, one
listing the IP address and the other listing the corresponding MAC (Ethernet)
address. When translating an IP address to an Ethernet address, the table is
searched for the row corresponding to the destination IP address, and the
corresponding Ethernet address is then found in the same row.
Whenever a packet must be sent to a new IP destination, that is, a destination
whose MAC address is not known, the IP host sends an ARP request packet,
listing its own IP address and MAC address, the destination IP address, but no
destination MAC address. When the packet reaches the destination address
(using the IP routing process), the destination returns an ARP response packet, in
which its own MAC address field is filled. The packet eventually returns to the
sender, thereby providing the missing information.