RFC 1868 - ARP

How does ARP work if hosts connecting through modems to their service providers are assigned addresses on the same subnet?  How does ARP work in this case?

And what if the service provider has multiple servers?
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ARP is a broadcast protocol. If a host wants the mac address of a device on it's own network it broadcasts an arp with the ip address it needs to find out. The device then merely responds with it's mac address.
If the device is on another subnet the arp is broadcast with the ip address of the gateway. When it receives this it constructs it's datagram with the ip address of the device it want's to communicate with but with the mac address of the router.
It should be the same for all implementations.

Hope that helps,

ARP - Address Resolution Protocol resolves 32 bit IP address to a 48 bit data link layer addresses specified in RFC 826.

there are two messages:
ARP query: Msg sent to data link layer broadcast address indicating a network layer address for which the transmitter seeks to a corresponding data link layer address. ARP query is broadcast so it goes to all nodes over Link L (IP and NON-IP) all nodes throw the query away except the network layer address host- who sends the response.
ARP response: Msg transmitted in response to an ARP query by the node whose network layer address was queried. It contains the desired data link layer address and is transmitted to the unicast layer 2 address that the ARP query was received.

When the node receives the response it makes an entry <D,DL> in its ARP cache.
D= Network Layer address
DL= Data Link Layer address
for further communication use.


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