Introduction
ARP—Address Resolution Protocol
A protocol called ARP (Address Resolution Protocol) is used to determine the MAC address of a particular IP address. The MAC (Medium Access Control) address is unique predefined number for each device on the LAN. The manufacturer of the device assigns MAC addresses. The IP address for each device is assigned by the network administrator according to the network structure.
If the originating station does not know the MAC address of the destination station, a MAC broadcast will be transmitted onto the LAN asking “Who has IP address 170.22.10.4?” This MAC broadcast is called an ARP request. Because the ARP request is a MAC broadcast, every device on the LAN will see the frame. The device that has the IP address 170.22.10.4 will respond with a frame to the originating station. The ARP reply frame will include the MAC address of the destination device.
Now when the two devices wish to send data across the LAN to each other, they will both use the MAC and IP address of the other device.
Each device on the LAN maintains a table for MAC addresses and IP addresses called the ARP cache. The ARP cache contains a list of IP addresses and their corresponding MAC addresses.
Proxy ARP
Each time an originating station does not know the MAC address of a destination station, the originating station sends out an ARP request. If the destination station is on a different network, the router connected to the originating network will see from the IP address that the frame is to be routed to another network. If the router has an entry for the destination address, the router will generate an ARP reply to send back to the originating station. The ARP reply will specify the MAC address of the router as the MAC address to send frames to for the IP address of the destination station.
The Complete IP Connection
The following are the steps that a frame of data will take when being transmitted from an originating station on an IP network to a destination station on a different IP network. In this example, the two networks are separated by a third network with two router hops between the originating network and the destination network.
•Originating station will send an ARP request if it does not have the MAC address of the destination station.
•Local router will see ARP request and send an ARP reply to the originating station with the MAC address of the local router port.
•Originating station will send the data frame addressed to the IP address of the destination station, and the MAC address of the local router port.
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