The principal packet structure of ARP packets is shown in the following table which illustrates the case of IPv4 networks running on Ethernet. The payload of the packet consists of four addresses, the hardware and protocol address of the sender and receiver hosts. The message header is completed with the operation code for request (1) and reply (2). The message header specifies the types of network in use at each layer as well as the size of addresses of each. The size of the ARP message depends on the link layer and network layer address sizes. In the case of Ethernet, a 0x0806 EtherType value is used to identify ARP frames. The packets are carried at the data link layer of the underlying network as raw payload. The Address Resolution Protocol uses a simple message format containing one address resolution request or response. It is communicated within the boundaries of a single network, never routed across internetworking nodes. Its messages are directly encapsulated by a link layer protocol. The Address Resolution Protocol is a request-response protocol.
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