The Complete Guide to ARP

What is ARP: Complete Explanation

ARP is short for Address Resolution Protocol. It is the mapping procedure that allows for an Internet Protocol (or IP) address to connect with a computer’s physical address (also known as a media access control, or MAC) when using a local area network. Read on to access the complete guide to ARP.

The entire procedure is necessary because computers will use different types of addresses when using a network. IP and MAC addresses use different formats: Internet Protocol addresses are 32 bits, but MAC addresses are 48 bits. As such, an Address Resolution Protocol will put both of these addresses in the same format and allow the two types of addresses to connect. 

It is important to note that all ARP’s are not the same. There are many different types of ARPs, including a Proxy ARP, Gratitutious ARP, Reverse ARP, and Inverse ARP. All of these ARPs can be used in different situations depending on a variety of factors, like whether or not the IP address is on the same network as the MAC address and whether or not one address is actively attempting to find the other address. 

ARP: An Exact Definition

An ARP is a procedure by which a computer’s Internet Protocol (IP) address can connect to a permanent physical machine address or media access control (MAC). This applies only to local area networks. They are commonly used in connections that take advantage of IPv4, or Internet Protocol Version 4.

ARP path switches
A diagram showing a path set up in a data center network of ARP Path switches. ARP will connect an IP and MAC when they are both trying to utilize the same local area network.

How Does ARP Work?

As noted above, an ARP will connect an IP and MAC when they are both trying to utilize the same local area network. This is necessary because each computer uses a different link-layer address. Specifically, Internet Protocol computers use a link-layer address that is somewhere on level 2 of the Open Systems Interconnection model, but a Media Access Control computer will use a link-layer address on level 3. As such, the specific goal of an ARP is to connect these two layers. Each of these link-layer addresses is represented by different addresses: Internet Protocol uses 32 bits, while Media Access Control uses 48 bits. 

ARP may be phased out as more networks upgrade to Internet Protocol version 6, which uses Neighbor Discovery protocol to connect between link-layer addresses.

An ARP comes into play when a computer using an IP address first attempts to join a LAN. That access will be stopped at a gateway, at which point the ARP program will kick into gear, finding a MAC address that will correspond with the appropriate IP address. If a computer is attempting to access the LAN for the first time, the ARP will get a new MAC address. If it is not the first time, it will reuse the same address as has been done previously. 

An ARP utilizes many specific mechanics, including:

Internet Protocol Address

An Internet Protocol (IP) address is the specific numerical address that a computer uses in order to connect to the internet. It can also be thought of as a computer’s “phone number” when trying to connect to a broader network. An IP address will identify a host and its location. In a sense, it is both a name and an address.

Media Access Control Address

A Media Access Control (MAC) address is a network address and is used in order to allow a computer to connect to a network. 

Local Area Network

A Local Area Network (LAN) is a network of computers that are connected together. They can be connected in any number of places, including a home or office. These networks can be very small and just consist of a couple of devices, or they can be very large and consist of thousands of computers that are using an office LAN together. 

IPv4

IPv4, first introduced in 1981, is the fourth version of Internet Protocol. IPv4 sets the technical boundaries for how computers access the internet and connect with each other. ARP occurs over IPv4, and while the specific program has been phased out in IP4v’s successor (IPv6), its functions are still completed by Neighbor Discovery. 

Where Did ARP Originate From?

ARP’s were first defined in 1982 when the process by which they would be used was created.  They began to be used during that time, with levels of use increasing as the internet became more and more commonly used. 

What Are The Applications of ARP?

The applications of ARP are relatively straightforward: An ARP is necessary for one scenario, and that is to connect an IP address to any Local Area Network. The different kinds of ARP – Proxy, Gratitutious, Reverse, and Inverse – also have different applications:

  • Proxy ARPs are used when an ARP is requested from a computer outside of the local area network in order to connect to the local area network.
  • Gratuitous ARPs are used by the ARP in order to determine the IP address. This happens in one of two scenarios: There is a potential security threat and the ARP’s programming has been triggered to defend the MAC address or to determine if the IP address in question is already in use.
  • Reverse ARP: As the name would imply, this ARP is used when the LAN is trying to determine an ARP’s IP address. Usually, this works in the opposite direction – hence the name.
  • Inverse ARP: Used to determine the IP address from its MAC address. 
ARP Network diagram
A diagram showing laptops connecting to a router and then to the internet. Using ARP is a requirement for any computer attempting to connect to a LAN.

Examples of ARP in the Real World

As noted above, an ARP is a requirement for any computer that is attempting to connect to a local area network. The process works as follows:

  • A Computer (let’s call it Computer 1) will attempt to connect to a local area network. It will do so by sending out an ARP request that will include its IP address.
  • This connection effort will accompany an ARP request. Using the ARP, the network will search for a matching MAC address.
  • Once that MAC address is found, it will be sent back to Computer 1, allowing for the connection to the LAN to be completed.

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