ARP stands for Address Resolution Protocol. ARP acts as one of the main protocols in the TCP / IP suite. It acts as a network protocol used to find out the MAC address of a device from an IP address. Therefore, in simple terms we can say, it translates a logical address to the machine’s physical address.

Definition of address

The address resolution protocol was devised to enable the communications on an inter-network. It is a network layer protocol which resides in the network layer of the OSI model. Therefore, ARP is a dynamic mapping protocol where each host in the network knows the logical address of another host. When a host needs to send an IP datagram to another host it must know the other host physical address. The data link layer does not understand the physical address because of the IP datagram is encapsulated in a frame so that it can pass through the physical layer. For this reason, network layer devices need ARP to map the IP network addresses to MAC hardware addresses. So, the sender would identify to which receiver the packet belongs.

Every host and router on the network receives and process the ARP request packet. But only the intended receiver recognizes its IP address in the request packet and sends back an ARP response packet.

ARP Protocol:

ARP accepts the logical address of the IP protocol, which maps the address to the appropriate physical address and forwards it to the data link layer.

Address Resolution Protocol Catching:

ARP is the process of storing network addresses and related data-link addresses in memory for a certain period of time. This also reduces the waste of network resource usage and requires updated cache entries. There are two types of cache entries, one static and one dynamic.

Proxy ARP:

The proxy poet was enforced to modify devices that square measure divided into physical network segments connected by a router within the same IP network or subnetwork to resolve IP-to-MAC addresses.

ARP Message Format:

The format of the ARP message is designed to adapt to layer two and layer three directions of various sizes. This diagram shows the most common implementation, which uses 32 bits for layer three addresses (“Protocol”) and 48 bits for layer two hardware addresses. Of course, these numbers correspond to the address sizes of the Internet Protocol version 4 and IEEE 802 MAC addresses used by Ethernet.

ARP packet format

Hardware Type:

This field specifies the type of hardware used for the local network that transmits the ARP message. Therefore, it also identifies the type of address used.

Protocol Type:

This field is a complement to the Hardware Type field, which specifies the type of address of the three layers used in the message. For IPv4 addresses, this value is 2048 (0800 hex), which corresponds to the EtherType code for Internet Protocol.

Hardware Address Length:

The length of the hardware address in the Address Resolution Protocol (ARP) message is the length in bytes of a hardware address (MAC). Ethernet MAC addresses are 6 bytes long.

Protocol Address Length:

Again, the complement of the previous field; determine how long the protocol address (layer three) in this message. For IP addresses (v4) this value is of course 4.

Op Code:

The opcode field in the Address Resolution Protocol (ARP) message determines the nature of the ARP message. 1 for ARP requests and 2 for ARP replies.

Sender Hardware Address:

The hardware (layer two) address of the device sending this message(which is the source device of the IP datagram in a request and the destination of the IP datagram in response, as explained in the topic on the ARP operation).

Sender Protocol Address:

The protocol address (IPv4 address) of the device that sends the message.

Target Hardware Address:

The hardware address (layer two) of the device to which this message can be sent. This is the destination device of the IP datagram in a request, and the origin of the IP datagram in a response).

Target Protocol Address:

The IP address of the device to which this message can be sent.

ARP acts as the fundamental protocol on the network today and there is abstraction between the MAC and IP addresses. Understanding ARP makes many network situations much clearer. This is one of the things you need to understand to be in the top 10% of IT professionals.