Address Resolution Protocol (ARP): Understanding the basics

Address resolution Protocol or ARP for short is basically used to resolve an IP address to a MAC address.
A MAC address is a unique identifier for any device. And since we are talking networking today, The mac address we will be concentration on is that of a network device.
Its important to note that, no two device can have the same mac address.


Whenever a network device needs to communicate with another network device in a Local Area Network, it needs the MAC address of that device. This is where ARP comes to play.

Lets take a look at the scenario bellow to clearly understand this concept:

1. There are 2 computers say A and B on the network.
2. They have both been assigned an IP address as seen below:
       A: 192.168.0.1
       B: 192.168.0.2
3. Computer A needs to communicate with computer B.
4. It looks at a list for the IP address to see if it has a corresponding MAC address. This list is called an ARP Cache.
5. If the entry is not in the ARP cache, it will attempt to create it by doing the following below
6. Computer A will broadcast the IP address over the network to locate Computer B and request for the mac address.
7. Computer B acknowledges and responds back with the mac address.
8. The IP address and corresponding MAC address will then be stored in the ARP cache.
9. Connection is completed and communication is established.

The following should be noted:
1. An IP address is used to Locate a devices while
2. A MAC address is used to identify the device.

How to check the ARP Cache
There is a utility on windows that can be used to see the contents of the ARP cache.
Lets go through a short step to see this:
1. Open Command prompt
2. type the following command and hit the enter key:
arp -a

It should be noted that the major advantage of the ARP cache is simply to make networking more efficient. What this means is that if the information needed to establish a connection is already contained in the ARP cache, then there is no need for the broadcasting of IP address on the network to request for MAC addresses. The connection is just simply established. Now that is a faster an efficient approach.

What are the contents of the ARP cache
Lets have a brief look at the contents of the ARP cache:
1. Internet address: This column stores the IP address of the network device
2. Physical Address: This column stores the MAC address of the device
3. Type: There are 2 different type of entries for this column (DYNAMIC and STATIC)

DYNAMIC
Dynamic is automatic and used only when the broadcast over the network for a MAC address is initiated and completed.
They are not permanent entries and thus are flushed out periodically so that the cache is not filled up with entries that are not been used.

STATIC
Static type is when the ARP cache details are entered manually by the network administrator through the ARP command line utility.

Advantage of Static over Dynamic
1. Static ARP cache is used over Dynamic to reduce unnecessary broadcast over the network.
2. Additionally its also a quick was to establish a connection if two computers are to constantly communicate with each other.

A static entry can be created using the follwing command:

arp -s {IP address} {MAC address}

An example can be seen below

arp -s 192.168.0.123 ff:02:f4:00:00


And that's it for this post. I hope you found it educative