Protocol length (PLEN) Length (in octets) of internetwork addresses. The internetwork protocol is specified in PTYPE. Example: IPv4 address length is 4. Operation Specifies the operation that the sender is performing: 1 for request, 2 for reply. Sender hardware address (SHA) Media address of the sender. In an ARP request this field is used to indicate the address of the host sending the request. In an ARP reply this field is used to indicate the address of the host that the request was looking for. Sender protocol address (SPA) Internetwork address of the sender. Target hardware address (THA) Media address of the intended receiver. In an ARP request this field is ignored. In an ARP reply this field is used to indicate the address of the host that originated the ARP request. Target protocol address (TPA) Internetwork address of the intended receiver. ARP protocol parameter values have been standardized and are maintained by the Internet Assigned Numbers Authority (IANA). The EtherType for ARP is . This appears in the Ethernet frame header when the payload is an ARP packet and is not to be confused with PTYPE, which appears within this encapsulated ARP packet. Layering ARP's placement within the Internet protocol suite and the OSI model may be a matter of confusion or even of dispute. mentions ARP within its link layer section without explicitly placing it within that layer. Some older references place ARP in OSI's data link layer while newer editions associate it with the network layer or introduce an intermediate OSI layer 2.5. Example Two computers in an office (Computer 1 and Computer 2) are connected to each other in a local area network by Ethernet cables and network switches, with no intervening gateways or routers. Computer 1 has a packet to send to Computer 2. Through DNS, it determines that Computer 2 has the IP address . To send the message, it also requires Computer 2s MAC address. First, Computer 1 uses a cached ARP table to look up for any existing records of Computer 2s MAC address (). If the MAC address is found, it sends an Ethernet frame containing the IP packet onto the link with the destination address . If the cache did not produce a result for , Computer 1 has to send a broadcast ARP request message (destination MAC address), which is accepted by all computers on the local network, requesting