What is Address Resolution Protocol (ARP) and How It Works? – POFTUT

What is Address Resolution Protocol (ARP) and How It Works?

Address Resolution Protocol (ARP) is auxiliary protocol used by Internet Protocol (IP). ARP is defined with RFC826 and gained main stream over computer networks. ARP is used to resolve IP address with MAC or hardware address of the network interface. As we know MAC address is used to identify nodes in a local network.

How ARP Works

ARP resolution works very simple way. The source creates requests to get MAC address of the destination system. And the request is replied by the destination system by providing its IP address. In the following example we will examine a request for IP address and reply from this IP address.

We will use following command in a Ubuntu linux box to print ARP packets in a verbose manner.

$ sudo tcpdump arp -XXX
How ARP Works
How ARP Works

We can see following conversation.

ARP, Request who-has tell ubuntu
ARP, Reply is-at 00:50:56:ea:83:e2

Protocol Resolution Types

As ARP is a simple protocol there is very little Protocol Resolution Types. We have all ready examined two of them.

  • ARP Request
  • ARP Response
  • RARP Request
  • RARP Response

We will detailed the RARP in the following parts.

Gratuitous ARP

ARP protocol can be used for different purposes. One of them is to defend and own selected IP address. A node can check its previous IP address with a gratious ARP request. This will create an ARP request for its own IP address.

Proxy ARP

Proxy ARP is used to relay ARP packets to the other segments of the network. Normally ARP will be broadcasted to the current network segment but this may limit ARP request in some network topology where L2 segmentation exist. We can configure gateway or similar devices to relay ARP and act like a Proxy to the other part or segment of the network. Proxy ARP is considered a security issue because of the original node do not answer the ARP request.

Reverse ARP or RARP

In a normal scenario ARP is used to resolve MAC address by using IP address. But in some cases we may need to resolve IP address from MAC address. This can be accomplished by using Reverse ARP a.k.a. RARP. As stated previously there is Request and Response which is the same of the ARP protocol .

  • RARP Request
  • RARP Response
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