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CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 45

Cisco Networking Academy Program CCNA 1 and 2 Companion Guide, Revised part 45 is the Cisco approved textbook to use alongside version 3.1 of the Cisco Networking Academy Program CCNA 1 and CCNA 2 web-based courses. The topics covered provide you with the necessary knowledge to begin your preparation for the CCNA certification exam (640-801, or 640-821 and 640-811) and to enter the field of network administration. | B 1102.book Page 409 Tuesday May 20 2003 2 53 PM IP Address Assignment Acquisition and Hierarchy 409 Figure 7-38 ARP Obtains the IP Address Via the MAC Address Host Y Broadcast ARP Request Host Z MAC Host Z Host Z ARP Reply Example 1 TCP IP Destination Local Host Y MAC Host Z MAC Net for Routing Table Host Z Example 2 TCP IP Destination not Local Table 7-10 ARP Table Entry Internet Address Physical Address Type 68.2.168.1 00-50-57-00-76-84 Dynamic Table 7-11 ARP Table for 198.150.11.36 MAC Address IP Address FE ED F9 44 45 66 198.150.11.34 DD EC BC AB 04 AC 198.150.11.33 DD EC BC 00 94 D4 198.150.11.35 FE ED F9 23 44 EF 198.150.11.36 Whenever a network device wants to send data across a network it uses information provided by its ARP table. In Figure 7-39 a source device wants to send data to another device. 1102.book Page 410 Tuesday May 20 2003 2 53 PM 410 Chapter 7 TCP IP Protocol Suite and IP Addressing Figure 7-39 ARP Tables ARP Table Physical Address IP Address 02-60-8C-01-02-03 197.15.22.33 00-00-A2-05-09-89 197.15.22.44 09-00-20-67-92-89 197.15.22.123 08-00-02-90-90-90 197.15.22.4 Destination 197.15.22.126 Source 197.15.22.4 197.15.22.33 197.15.22.44 197.15.22.123 197.15.22.37 197.15.22.126 Destination ARP Operation Within a Subnet If a host wants to send data to another host it must know the destination IP and MAC addresses. If it cannot locate a MAC address for the destination IP address in its ARP table the host initiates a process called an ARP request as shown in Figure 7-39. An ARP request lets a host discover the destination MAC address. A host builds an ARP request packet and sends it to all devices on the network. This ARP request packet is divided into two parts The frame header The ARP message To ensure that all devices see the ARP request the source uses a broadcast MAC address. The broadcast address in a MAC addressing scheme has all places set to hexadecimal F. Thus a MAC broadcast address has the form FF-FF-FF-FF-FF-FF. Because ARP request .