What is the ARP (Address Resolution Protocol)?

Address Resolution Protocol: In order to send data packets in TCP / IP networks, a server needs, above all, three address data on the host to which it addressed: the subnet mask, the IP address and the MAC address (also known as the physical address or hardware address). The devices receive the netmask and IP address automatically and flexibly when the connection to a network will establish. With this objective, mediating communication devices such as routers or hubs use the DHCP protocol. Both local data can enter manually. The corresponding device manufacturer grants the hardware address, which it linked to an IP address with the help of the so-called Address Resolution Protocol (ARP).

Address Resolution Protocol Definition

The Address Resolution Protocol as specified in 1982 in the RFC 826 standard to carry out the resolution of IPv4 addresses in MAC addresses. ARP is essential for the transmission of data in Ethernet networks for two reasons: on the one hand, data frames (also Ethernet frames) of IP packets can only be sent with the help of a hardware address to the destination hosts, but the Internet protocol cannot get these physical addresses by itself. On the other, and due to its limited length, the IPv4 protocol lacks the ability to store device addresses. With a cache mechanism own, the ARP protocol is also, here, the most appropriate solution. IPv6, for its part, adopts the functions of the Neighbor Discovery Protocol (NDP).

How Address Resolution Protocol Messages Created

The ARP protocol uses a simple message format to send or respond to requests. Although initially it was intended for IPv4 addresses and MAC addresses, other network protocols are also valid. Hence there are fields for the type and size of hardware and protocol addresses. As a consequence, there may also be differences in the total size of the ARP packets. In typical situations, the use of the ARP protocol in networks based on Ethernet and IPv4 results in, for example, a length of 224 bits (28 bytes).

The ARP header begins with the 16-bit length information about the type of hardware address. In the case of the planned Ethernet devices, the package would have the value 1. The protocol indicated below (16 bits too), which should serve as the basis for the resolution of ARP addresses. IPv4 addresses are distinguished by the value 0x0800 (2048), which in fact is also used in Ethernet frames as a value for the IPv4 (Ether Type) protocol.

The next 16 bits (operation) reserved for the ARP message specification: the value 1 used for an ARP request, and 2 reveals that it is an ARP response. Finally, the packages receive the four relevant and previously declared addresses:

1, MAC address of the sender:

An entry contains the MAC address of the requesting computer; in this ARP response, the physical destination address or the next gateway is in this case.

2, Sender IP Address:

Contains the IP address of the requesting computer (ARP request) or destination or the next gateway (response).  

3, Recipient’s MAC address:

This field shows the broadcast MAC address FF: FF: FF: FF: FF: FF when the sender makes a request. The requesting host of the hardware address is recorded in the ARP response.

4, Recipient IP Address:

In a request, this field includes the host IP address and the MAC address will have to be searched. The reply message contains the IP address of the requesting computer.

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