The Address Resolution Protocol was specified in 1982 in the RFC 826 standard to carry out the resolution of IPv4 addresses to MAC addresses. ARP is essential for data transmission on Ethernet networks for two reasons: on the one hand, data frames (also Ethernet frames) of IP packets can only be sent with the aid of a hardware address to the destination hosts, but the Internet Protocol cannot obtain these physical addresses by itself. On the other hand, and due to its limited length, the IPv4 protocol lacks the ability to store device addresses. With its own caching mechanism, the ARP protocol is also the most suitable solution here. IPv6, for its part, adopts the functions of the Neighbor Discovery Protocol (NDP).
Unlike most protocols, ARP packets do not have fixed-format headers. Instead, the message is designed to be useful with different network technologies, such as:
▸Ethernet LAN adapter (supports Ethernet and 802.3 protocols)
▸Ring Network Adapter
▸FDDI network adapter
However, ARP does not convert addresses for SLIP or SOC since these are point-to-point connections. In addition, when assigning addresses by means of the Address Resolution Protocol, it is necessary to distinguish if the IP address of the destination host is in the same local network or in another subnet. Thus, in case of assigning a MAC address to a certain IP address, first of all a review of the subnet mask is carried out. If the IP is on the local network, the first step is to check whether an entry for it already exists in the ARP cache.
The ARP protocol uses a simple message format to send or respond to requests. Although initially 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.
However, we must remember that the ARP protocol is nothing more than a simple message format that has an address resolution request or response. In this sense, the size of the ARP message depends on the lower and upper strata in size of the layer address, which is conditioned by the type of network protocol being used (generally IPv4), as well as the kind of “virtual link layer that the upper layer protocol runs on the hardware.”
The structure of the ARP could be defined as:
▸Hardware type
▸Protocol type
▸Hardware Length
▸Protocol length
▸Operation
▸Sender Hardware Address
▸Sender protocol address
▸Destination Hardware Address
▸Target Protocol Address
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