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Networks Basics: Address Resolution Protocol (ARP)

ARP, so called Address Resolution Protocol is a layer 3 protocol and is essential to permit data used in making a layer 2 frame comprehensive with destination and source MAC addresses. 

Every router keeps a table with register IP addresses and particular hardware addresses which we called as MAC addresses of devices that occur on the network. 

The table is called an ARP cache and is referenced by the router when it is considering up a hardware address of a device for which it recognizes the IP address and wants to forward a datagram to it. 

Fig 1.1- Address Resolution Protocol

If no hardware address is located in the ARP cache then an ARP broadcast is sent on to the contiguous media. This transmission is read by every place comprising the target destination.

The target destination directs in return an ARP response with its hardware address so that the IP datagram can instantly be forwarded to it by the router. 

Although ARP is a layer 3 protocol and it does not use an IP header, it has its peculiar packet format and is a broadcast on the local LAN within the data field of a layer 2 frame without needing to be routed. 

The Ethernet Type field has the value 0x0806 to indicate an ARP. The following diagram details the ARP packet

  • Hardware Type: Hardware type for ARP is 1 for Ethernet.
  • Protocol Type: It’s a Layer 3 protocol and named as Network layer protocol.
  • Hardware Address Length: The Hardware address length is 6 bytes for Ethernet packet.
  • Protocol Address Length: The Protocol address length is 4 bytes for TCP/IP
  • Operation Code: It can be ARP appeal or reply
  • Senders Hardware Address: Source node hardware address
  • Senders Protocol Address: It will be layer 3 address.
  • Target Hardware Address: The Target Hardware address generally used in Reverse ARP, which carries both the target’s MAC and layer 3 IP addresses.
  • Target Protocol Address: The Target Protocol address is used in an ARP request, the response carries both the target’s MAC and layer 3 IP addresses.

Author: Amandeep Kaur, Network Engineer

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