Definition Of Address Resolution Protocol

The Address Resolution Protocol is a communication protocol used for discovering the link layer address, such as a MAC address, associated with a given internet layer address, typically an IPv4 address. This mapping is a critical function in the Internet protocol suite. 

Address Resolution Protocol (ARP) is a protocol or procedure that connects an ever-changing Internet Protocol (IP) address to a fixed physical machine address, also known as a media access control (MAC) address, in a local-area network (LAN). 

This mapping procedure is important because the lengths of the IP and MAC addresses differ, and a translation is needed so that the systems can recognize one another. The most used IP today is IP version 4 (IPv4). An IP address is 32 bits long. However, MAC addresses are 48 bits long. ARP translates the 32-bit address to 48 and vice versa.

What Are the Types of ARP?

There are different versions and use cases of ARP. Let us take a look at a few.

Proxy ARP

Proxy ARP is a technique by which a proxy device on a given network answers the ARP request for an IP address that is not on that network. The proxy is aware of the location of the traffic’s destination and offers its own MAC address as the destination. 

Gratuitous ARP

Gratuitous ARP is almost like an administrative procedure, carried out as a way for a host on a network to simply announce or update its IP-to-MAC address. Gratuitous ARP is not prompted by an ARP request to translate an IP address to a MAC address.


Reverse ARP (RARP)

Host machines that do not know their own IP address can use the Reverse Address Resolution Protocol (RARP) for discovery.

Inverse ARP (IARP)

Whereas ARP uses an IP address to find a MAC address, IARP uses a MAC address to find an IP address.

How Does ARP Work?

When a new computer joins a LAN, it is assigned a unique IP address to use for identification and communication. 

Packets of data arrive at a gateway, destined for a particular host machine. The gateway, or the piece of hardware on a network that allows data to flow from one network to another, asks the ARP program to find a MAC address that matches the IP address. The ARP cache keeps a record of each IP address and its matching MAC address. The ARP cache is dynamic, but users on a network can also configure a static ARP Table containing IP addresses and MAC addresses.

ARP caches are kept on all operating systems in an IPv4 Ethernet network. Every time a device requests a MAC address to send data to another device connected to the LAN, the device verifies its ARP cache to see if the IP-to-MAC-address connection has already been completed. If it exists, then a new request is unnecessary. However, if the translation has not yet been carried out, then the request for network addresses is sent, and ARP is performed.

An ARP cache size is limited by design, and addresses tend to stay in the cache for only a few minutes. It is purged regularly to free up space. This design is also intended for privacy and security to prevent IP addresses from being stolen or spoofed by cyberattackers. While MAC addresses are fixed, IP addresses are constantly changing.

In the purging process, unutilized addresses are deleted; so is any data related to unsuccessful tries to communicate with computers not connected to the network or that are not even powered on.

How to Configure the Address Resolution Protocol

By default, the Address ResolutionProtocol (ARP) feature is enabled and is set to use Ethernet encapsulation.
Perform the following tasks to change or verify ARP functionality:
Enabling the Interface Encapsulation
Perform this task to support a type of encapsulation for a specific network, such as Ethernet, Frame Relay,
FDDI, or Token Ring. When Frame Relay encapsulation is specified, the interface is configured for a Frame
Relay subnetwork with one physical link that has many logical circuits called virtual circuits (VCs). The
address field in the frame contains a data-link connection identifier (DLCI) that identifies each VC. When
SNAP encapsulation is specified, the interface is configured for FDDI or Token Ring networks.
The encapsulation type specified in thistask should match the encapsulation type specified in the “Defining
Static ARP Entries” task.

  1. enable
  2. configure terminal
  3. interface type number
  4. arp {arpa | frame-relay | snap}
  5. end
    Command or Action Purpose
    Step 1 enable Enables privileged EXEC mode.
    Device> enable
    • Enter your password if prompted.
    configure terminal Enters global configuration mode.
    Device# configure terminal
    Step 2
    interface type number Enters interface configuration mode.
    Device(config)# interface
    Step 3
    IP Addressing: ARP Configuration Guide, Cisco IOS XE Release 3SE (Catalyst 3650 Switches)
    Address Resolution Protocol
    How to Configure the Address Resolution Protocol
    Command or Action Purpose
    Specifiesthe encapsulation type for an interface by type of network,
    such as Ethernet, FDDI, Frame Relay, and Token Ring. The
    keywords are as follows:
    arp {arpa | frame-relay | snap}
    Device(config-if)# arp arpa
    Step 4
    • arpa—Enables encapsulation for an Ethernet 802.3 network.
    • frame-relay—Enables encapsulation for a Frame Relay
    • snap—Enables encapsulation for FDDI and Token Ring
    end Returns to privileged EXEC mode.
    Device(config-if)# end
    Step 5
    Defining Static ARP Entries
    Perform this task to define static mapping betwee

Why is ARP Necessary?

ARP is necessary because the software address (IP address) of the host or computer connected to the network needs to be translated to a hardware address (MAC address). Without ARP, a host would not be able to figure out the hardware address of another host. The LAN keeps a table or directory that maps IP addresses to MAC addresses of the different devices, including both endpoints and routers on that network.

This table or directory is not maintained by users or even by IT administrators. Instead, the ARP protocol creates entries on the fly. If a user’s device does not know the hardware address of the destination host, the device will send a message to every host on the network asking for this address. When the proper destination host learns of the request, it will reply back with its hardware address, which will then be stored in the ARP directory or table. 

If ARP is not supported, manual entries can be made to this directory. 

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