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Network Basics: Remote Host ARP Requests

If your targeted host is not on the local subnet it is considered a remote host. The Address Resolution Protocol (ARP) the process of establishing a remote host connection is illustrated in the following figure.

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The process starts with an ARP to the default gateway that triggers a series of ARPs from that gateway or router through all the routers that connect these two hosts. For simplicity, the figure shows only one router, but there could be any number of routers between these two hosts. The following steps explain this process:

  1. The first host has data to send to the second host, the data is placed on hold, and two AND operations are performed.

    One operation uses the IP address and subnet mask of the first host, and the second uses the IP address of the second host and the subnet mask of the first host.

    The result is that two hosts are on separate network segments.

  2. A communication session needs to be established through the network routers.

    The closest router that is known is the default gateway (192.168.1.1). To be able to send data through the router, the first host needs to know the MAC address of the router. The host checks the ARP cache, and if the MAC address for 192.168.1.1 is not there, it sends an ARP request for the 192.168.1.1 IP address.

  3. After the first host gets the ARP reply from the router, the first host releases the data that needs sent to the second host.

    Because the data is going to a host on a remote network segment, the data is sent through the router.

  4. The data arrives at the router, and the router determines whether the second host is local to any of its attached network interfaces or network segments.

  5. If the second host is on a connected network segment, the router can send an ARP request looking for the MAC address of the host with the 10.45.11.54 IP address; however, if the second host is not on a connected network segment, the router needs to send the ARP request to another router that the router thinks is closer to the second host.

    In this case, the second host is directly attached to the required network segment, and the router would know that by going through the AND operation for all its network interfaces and the IP address of the second host.

  6. After the router identifies the network connection or network interface where the router expects to find the second host, the router sends the ARP request to that network segment.

  7. The second host, as shown below, now knows that another host is attempting to communicate with it. The second host records the IP address and MAC information for the router in its ARP cache.

    However, the second host does not know the first host’s MAC address; it never needs to know this info.

  8. The second host sends the ARP reply back to the router on its network.

  9. After the router receives the ARP reply from the second host, it knows how to get the pending data to the second host, and that data is then sent over the shared network segment to the second host.

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