TAILIEUCHUNG - CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 46

Cisco Networking Academy Program CCNA 1 and 2 Companion Guide, Revised part 46 is the Cisco approved textbook to use alongside version 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. | Page 419 Tuesday May 20 2003 2 53 PM Summary 419 Routers running proxy ARP capture ARP packets. They respond with their MAC addresses for those requests in which the IP address is not in the range of addresses of the local subnet. For example in Figure 7-49 if Machine A sends an ARP requests to Machine F the router processes the request and replies to A with its own MAC address to be mapped to the IP address of Machine F. In the previous description of how data is sent to a host on a different subnet the default gateway is configured. If the source host does not have a default gateway configured it sends an ARP request. All hosts on the segment including the router receive the ARP request. The router compares the IP destination address with the IP subnet address to determine whether the destination IP address is on the same subnet as the source host. If the subnet address is the same the router discards the packet. The packet is discarded because the destination IP address is on the same segment as the source s IP address. This means that the destination device on the segment should respond to the ARP request. The exception to this is that the destination IP address is not currently assigned which generates an error response on the source host. Figure 7-49 Proxy ARP Network Example Router Summary In this chapter you learned the following key points To provide extra flexibility for the network administrator networks particularly large ones are often divided into smaller networks called subnetworks or subnets. Subnetting allows a network administrator to get around IPv4 s limitations by dividing a single network address into many subnets visible only within that single network. The function of a subnet mask is to tell devices which part of an address is the network number including the subnet and which part is the host. Internetworking functions of the network layer include network addressing and best-path selection for data traffic. B Page 420 .

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