ARP is short of Address Resolution Protocol and it is one of the most important parts of IP networking as it is used to connect OSI Layer 3 for Network to OSI Layer 2 for Data Link. This will mean that ARP is used to link our IP addressing to our Ethernet addressing also known as MAC addressing. For anyone to communicate with any device on a network they must have the Ethernet MAC address for that device. If this device is not on the Local Area Network you can go through the default gateway, which is the router, and this router will be the destination MAC address that your PC will communicate with. The ARP cache is a table that stores mappings between Data Link Layer addresses as well as Network Layer addresses.
The Data Link Addresses are usually MAC addresses while the Network Layer addresses are most frequently IP addresses. The Address Resolution Protocol cache is stored in Random Access Memory by the operating system. There are two types of ARP entries namely static and dynamic. In most cases the dynamic ARP entries are the ones that are used and this means that the ARP entry is kept on a device for some period of time so long as it is being used. The static ARP entries require the user to annually enter the link between the Ethernet MAC address as the IP address.
Dynamic ARP entries are not used most of the time due to the management headaches as well as the lack of significant negatives to using dynamic ARP entries. An ARP protocol is used to create a dynamic ARP entry. If a router is communicating with a device it always has its own Address Resolution Protocol cache. All the switches have their own cache but they will also keep track of which MAC address is connected to which port on the switch and this can be seen with the show MAC address table commands on a Cisco IOS switch.
There is also an Inverse ARP protocol that performs the opposite task that the ARP protocol performs. Inverse ARP asks all device s if they have the MAC address xxxx.xxxx.xxxx and, if so, what their IP address is. All the IP devices that are in the market today have a Address Resolution Protocol cache which can be used in your troubleshooting of network connectivity. If everything is working fine with ARP you will still have a dynamic ARP entry which is complete where both the MAC and IP values will be included.
For those who do not have a complete entry they can clear their Address Resolution Protocol cache and attempt communication again by pinging the device they are trying to communicate with. If you try to communicate with a Cisco router you can perform a show ARP then a clear ARP and attempt communications again to make sure the issue is resolved. The Microsoft Windows Address Resolution Protocol cache occasionally becomes corrupt and will need to be cleared. One of the first symptoms of this are connections to web pages timing out or failing.
Author Resource:
This article touched the basics of the topic. I have 2 more resources related to the above. They are arp cache and arp cache . They are worth a read.
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