Network Lay Computer Networks 4-90 ver * Use the command show history to see the last 10 commands entered on the router. ge Raker ab unary ‘ sh history pee wee uy) ‘sh cdp neig f ‘sh ver sh flash * The command show version will provide basic configuration for the system hardware as well as the software version, the names and sources of configuration files, and the boot images. & "/"Routername#ah version EEE] arp SEE * Since there is no dependence between local addresses and TP addresses, the only method of establishing the mapping is using tables. As a result of network configuration, each interface knows its local address and IP address. This mapping can be considered as a table distributed over individual network interfaces. Anytime a host or a router has an IP datagram to send to another host or router, it has the logical IP address of the receiver. But the IP datagram must be encapsulated in a frame to be able to pass through the Physical network. This means that the sender needs the physical address of the receiver. A mapping corresponds a logical address to a physical address. The mapping can be done dynamically, receiver to announce its physical addres: purpose. which means that the sender asks the s when needed. ARP is used for this ARP associates an IP address with its physical address, Anytime a host, or a Touter needs to find the physical address of another host or router on its network, it sends an ARP query packet. The packet includes the physical’ address and IP address of the sender and the IP address of the receiy Fig. 419.1 shows tho operpation of ARP. (See Fig. 4.19.1 on next Pare) * Consider the following, example, computer A and computer B share a physical network. Each computer hes an assigned IP address 1, and J Physical addresses " Wowout Reuomneg™ arena PUBLICA TrOW2? An up must fr trewtnize ee hE..." Computer Networks 4-91 ()Unleast rophy Fig. 4.19.1 ARP operation Py and Pg. The goal is to device low-level software that hides physical addre: and allows higher-level programs to work only with Internet addresses. Address mapping must be performed at each step along the path from the orig source to the destination. The sender must map the intermediate router's Inte address to a physical address. The problem of mapping high-level addresses Physical addresses is known as the address resolution problem and has b solved in several ways. Physical addresses are two types. 1. Ethemet 2, ProNET Ethernet has large and fixed physical addresses. ProNET has small, ca configured physical addresses. Address resolution is difficult for ethemet networks but easy for network like porNET. ARP allows a host to find physical address of a target host on the same physical network, given only target's IP address. TECHNICAL PUBLICATIONS® - An up thrust for knowedge rrComputer Networks 4-92 Network Layer » Fig. 4.19.1 (a) shows host A broadcasts an ARP request containing I, to all computer on the network and Fig. 4.19.1 (b) shows host B responds with an ARP replay that contains the pair (Ig, Py). To reduce communication costs, ARP maintain a cache of recently acquired IP to Physical address binding, so they do not have to use ARP repeatedly. Whenever a computer receives an ARP reply, it saves the sender's IP address and corresponding hardware address in its cache for successive lookups. When transmitting a packet, a computer always look in its cache for a binding before sending an ARP request. If a computer finds the desired binding in its ARP cache, it need not broadcast on the network. When ARP message travel from one computer to another, they must be carried in physical frame. Mg, 419-2scholée-that the ARP is ig. 419.2 shows that the ARP message cated in te data portion of a frame. | To identify the frame as carrying on ARP SrA ‘, * }. 4, message, sender assigns a special value to gnehiSulitO? AR, ARP message. the type field in the frame header, and Places the ARP message in the frame data field. The data in ARP packets does not have a fixed format header. EEEEI Packet Format * Fig. 4.19.3 shows the ARP packet format. be tts Hardware type Harcware Protocol length Sender hardware address ‘Target hardware address, ‘Target protocol address Fig. 4.19.3 ARP packet TECHNICAL PUBLICATIONS® « An up thst for knowedge aComputer Networks 4-93 Network Layer 1. Hardware type : This is 16 bits field defining the type of the network on which ARP is running. Ethernet is given the type 1. 2 Protocol type : This is 16 bits field defining the protocol. The value of this field for the IPv4 protocol is 0800H. 3. Hardware length : This is an 8 bits field defining the length of the physical address in bytes. Ethemet is the value 6. 4. Protocol length : This is an 8 bits field defining the length of the logical address in bytes. For the IPv4 protocol the value is 4. 5. Opertion : This is a 16 bits field defining the type of packet. Packet types are ARP request (1), ARP reply (2). 6. Sender hardware address : This is a variable length field defining the physical address of the sender. For example, for Ethernet this field is 6 bytes long. 7. Sender protacol address : This is also a variable length field defining the logical address of the sender. For the IP potocol, this field is 4 bytes long. 8. Target hardware address : This is a variable length field defining the physical address of the target. For Ethernet this field is 6 bytes long. For ARP request message, this field is all Os because the sender does not know the physical address of the target. 9. Target protocol address : This is also a variable length field defining the logical address of the target. For the IPv4 protocol, this field is 4 bytes long. EKER] Encapsulation * Fig. 4.194 shows the Etheret frame with an encapsulated ARP message. ARP request and reply have the same format. B bytes Bbyes Gbytes 2 bytes. A bytes Preamble srati Destination | Source ARP request or reply packet Fig, 4.19.4 Encapsulation of ARP packet TECHNIGAL PUBLICATIONS® - An up thrus! for knowledgeComputer Networks 44 Network Leyer <») Example of ARP. request sy Network type Operation = Local address of the sender Local address of the receiver e 1 Network... address receiver “Network adie ‘receiver ~~ ee * To redtice the ARP messages in the ‘network, the detected mapping between IP and MAC addresses is stored in the ARP table of the appropriate interface. In this case, this record ‘will appéar as follow: 172.16.16.30 : ~ OOEOF77F1920-'* LTECHNICAL PUBLICATIONS® » An up thrust for knowledige - — a